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Department of Cell Biology (S.M.M., P.A., G.S.M.,
L-y.Y-L.) Department of Microbiology and Immunology (L-y.Y-L.)
Department of Medicine (L-y.Y-L.) Baylor College of Medicine
Houston, Texas 77030
Department of Pharmacology (X.X.,
N.R.M.) University of Medicine and Dentistry of New Jersey
Robert Wood Johnson Medical School Piscataway, New Jersey 08854
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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Reorientation and movement of intracellular organelles, in particular, the centrosome MTOC, the nucleus, and the Golgi, depends on MT interactions with the cell cortex and is coordinated by forces acting on the centrosomes themselves (4, 5, 6). Perhaps the best evidence that there are forces acting directly on the centrosomes comes from studies of embryogenesis in Drosophila melanogaster. In the Drosophila embryo, nuclei migrate to the cortex before cellularization (7). If nuclear division is aborted, either by drug action or mutation, the centrosomes nevertheless continue to multiply and migrate to the cortex independently of the nuclei (8, 9). This provides strong evidence that nuclear migration is mediated through a force acting directly on the centrosome itself, which in turn serves to move the nucleus.
In simple eukaryotes such as fungi and yeast, nuclear movement is a MT-dependent process (10, 11). The main MT-dependent "motor" involved in centrosomal and nuclear positioning has been identified as cytoplasmic dynein. The evidence that cytoplasmic dynein is the main motor protein for nuclear positioning comes from the studies of the nudA, ro-1, and DYN1 genes, which encode the cytoplasmic dynein heavy chains of Aspergillus nidulans, Neurospora crassa, and Saccharomyces cerevisiae, respectively. Mutations in the nudA gene and the ro-1 gene cause a failure of nuclear movement through the germ tubes (12, 13). In A. nidulans, the dynein heavy chain has been localized to the tip of the growing germ tube (14). A mutation in the 8-kDa A. nidulans dynein light chain, encoded by the nudG gene, causes the heavy chain to be lost from the tip and also affects nuclear movement (15). Similarly, disruption of DYN1 causes a partial failure of nuclear segregation between mother and daughter cells in yeast. This failure to segregate is caused by a defect in the ability of mitotic nuclei to migrate to the correct orientation before anaphase (16, 17). Additionally, in yeast, MTs attached to the spindle pole body (SPB), the centrosome equivalent in lower eukaryotes, have been shown to be essential for the nuclear orientation process (18). Furthermore, several of the N. crassa ropy mutations have been shown to affect dynactin, a complex that is required for the coupling of dynein to its cargo, and result in a nuclear movement defect (13, 19, 20). In yeast, mutations have also been identified that affect nuclear positioning (21, 22, 23). Together these data support a model in which SPB orientation and nuclear movement are mediated via an interaction between SPB MTs and a dynactin-associated, minus end-directed dynein motor located at the cell cortex.
In filamentous fungi, e.g. A. nidulans and N. crassa, nuclear movement is required for normal colony formation (13, 24). During the germination of conidia (asexual spores), the parental nucleus divides and the daughter nuclei move out in the germ tube of the developing germling. Temperature-sensitive mutations have been identified in A. nidulans that affect this initial stage of nuclear movement as well as A. nidulans’ ability to maintain nuclei of additional nuclear divisions at a uniform distance from each other. Because of the observable defect in nuclear distribution, this class of mutants was call nud’s (24). In addition to nudA and nudG, two other genes have been identified, nudC and nudF (12, 24, 25, 26). The nudC gene encodes a 22-kDa protein of unknown function (25). The nudF gene encodes a 49-kDa protein that is similar to the human lissencephaly (LIS-1) protein (26). LIS-1 is involved in controlling neuronal migration in the cerebral cortex (27). Through A. nidulans studies, NUDC has been shown to posttranscriptionally regulate NUDF (26). Additionally, NUDF appears to be an upstream regulator of cytoplasmic dynein/dynactin function, as a newly discovered mutation in the cytoplasmic dynein heavy chain acts as a bypass suppressor of the nudF deletion (D. A. Willins and N. R. Morris, unpublished data). Therefore, through genetic studies, NUDC can be placed upstream of NUDF, and in turn both NUDC and NUDF can be placed upstream of dynein and dynactin.
The rat Nb2 T cell line can be stimulated to proliferate by the addition of 1–10 ng/ml PRL to the cell culture medium (28). One of the PRL-inducible genes cloned from Nb2 T cells is clone 15 (c15) (29). The c15 gene encodes a 332-amino acid (aa) protein (45-kDa) in which the carboxy-terminal 94 aa (Gly 239 to Asn 332) are 68% identical to the carboxy-terminal portion of the A. nidulans nuclear movement protein NUDC (Gly 105 to Gly 198). This striking similarity suggests that the dynein/dynactin pathway that mediates nuclear movement in fungi may also be involved in reorientation of the centrosomal MTs and Golgi in T cells, and that the c15 and nudC gene products may share similar functions. The present work was designed to test the hypothesis that c15 and NUDC are functionally related by determining whether the rat T cell c15 protein could complement the temperature sensitivity of the A. nidulans nudC3 mutation.
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RESULTS |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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) (31, 32). The H2A
3'-fragment allows for more efficient processing and export of the
mature mRNA from the nucleus and greater stability of the mRNA
transcript. The pAL5 vector contains, as does the pAL3 parental vector,
the alcA alcohol dehydrogenase I gene promoter, which is
inducible or repressible based on the growth media and carbon source
present. Transcription of the gene of interest is repressed when
A. nidulans containing this construct are grown on rich
glucose media (MAG or YAG), and expression is allowed when A.
nidulans are grown on glycerol-containing media (32, 33). In
addition, both pAL5 and pAL3 vectors contain the ampicillin resistance
gene, to allow for selection in E. coli, and the N.
crassa pyr4 gene. The pyr4 gene encodes
for orotidine 5'-P decarboxylase and is capable of complementing the
pyrG89 mutation in A. nidulans, thereby allowing
A. nidulans to grow in the absence of exogenous uracil and
uridine (30).
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). A.
nidulans containing the nudC3 temperature-sensitive
mutation were stably transformed with either the c15 vector or the pAL5
vector. After transformation, 11 c15 vector-transformed strains and 13
pAL5 vector-transformed strains were obtained and assayed for
complementation of the nudC3 mutation at the restrictive
temperature (42 C), on the appropriate media (glycerol).
c15 Complements nudC3 Mutants
The A. nidulans nudC3 mutant strain is temperature
sensitive (Fig. 2A). At the permissive temperature, 32
C, the colonies are similar to the wild type strain as they are large,
white, have undergone normal growth and differentiation, and are able
to generate spores. However, at the restrictive temperature, 42 C, the
nudC3 colonies are much smaller than the wild type colonies
grown at the restrictive temperature, brown in color, restricted in
cellular growth, and unable to undergo normal differentiation to form
conidia.
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). A.
nidulans containing either construct grew well on either of the
two different media at 32 C. However, when the transformants were grown
at the restrictive temperature, only those on the glycerol minimal
media containing the c15 construct grew. The c15 transformants grew in
a similar manner on glycerol minimal media at either 32 C or 42 C.
Therefore, the results of this experiment indicate that the rat c15
protein has the ability to complement the functions lost by the mutant
A. nidulans NUDC protein and restore normal growth and
differentiation.
c15 Protein Is Expressed in Complemented A.
nidulans
To verify that the c15 protein was being expressed in the
c15-transformed and -complemented A. nidulans, Western blot
analysis of the total cellular proteins was performed using
affinity-purified rabbit anti-c15-carboxy-peptide (c15-C) antibodies
(S. M. Morris, in preparation). The expression of the 45-kDa c15
protein (arrow) was detected in the two c15-complemented
strains tested (Fig. 3, lanes 2 and 3) and not in the
control nudC3 strain (Fig. 3, lane 1) or those strains
containing the pAL5 vector (Fig. 3, lanes 4 and 5). Additionally, the
level of c15 protein expression in the two different c15-transformed
strains varied. The c15#1 strain (Fig. 3, lane 2)
consistently expressed more c15 protein than did the c15#2
strain (Fig. 3, lane 3). Furthermore, the c15-C antibodies specifically
recognized a 22-kDa protein band (asterisk), corresponding
to the mutant NUDC protein in all protein preparations. Interestingly,
the overall levels of endogenous mutant NUDC in the c15-expressing
strains was lower than the levels of mutant NUDC found in the
nontransformed nudC3 strain or in the two pAL5-transformed
strains. Both the 45-kDa and 22-kDa protein bands could be specifically
competed by the addition of c15-C to the immunoblotting solutions (data
not shown).
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). In contrast, the nuclei of
A. nidulans containing the nudC3 mutation, when
grown at 42 C, divided but failed to migrate out into the germ tube
(Fig. 4C). When nudC3 mutant A. nidulans were
transformed with the c15 complementation construct and were grown at 42
C on glycerol minimal media, their phenotype closely resembled that of
the nontransformed nudC3 mutants grown at the permissive
temperature (32 C). The c15-complemented A. nidulans
extended and elongated their germ tube (Fig. 4E), and their nuclei
migrated into the germ tube of the developing germling.
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). In nudC3 mutants, grown at 32 C in
glycerol media, more than 95% of the germlings moved their nuclei. In
contrast, when nudC3 mutants were grown at 42 C in glycerol
media, only approximately 18% of the germlings counted showed any form
of nuclear movement. When the c15-transformed A. nidulans
were grown at 42 C in glycerol media, nearly 90% of the germlings
moved their nuclei. Therefore, c15 is capable of restoring normal
nuclear migration.
c15 Protein Expression Restores NUDF Expression
Endogenous levels of NUDF decrease when nudC3 mutants
are grown at the restrictive temperature of 42 C. Studies involving the
use of an exogenous promoter to induce nudF gene expression
revealed a similar decrease in overall NUDF expression when this
construct was transformed into nudC3 mutants that were grown
at 42 C. These results suggest that NUDC regulates NUDF levels in a
posttranscriptional manner (26). To determine whether or not the
expression of c15 at 42 C in nudC3 mutants could rescue NUDF
expression, Western blot analysis of total cellular proteins was
performed using affinity-purified anti-NUDF antibodies. The expression
of the 49-kDa NUDF protein was detected at normal levels in the wild
type strain (Fig. 5, lane 1) and in the nudA4
mutant strain (Fig. 5, lane 2) grown at 42 C. However, the expression
of the NUDF protein was greatly decreased in the nudF7
mutant strain and the nudC3 mutant strain (Fig. 5, lanes 3
and 4) grown at 42 C. When the c15 protein was expressed at 42 C, in
the two c15-complemented strains tested, the expression of NUDF was
restored (Fig. 5, lanes 5 and 6). The level to which NUDF protein was
restored correlates well with the level of c15 protein expression
observed in the two c15-transformed strains (Fig. 3). As a control for
the transformation, two control pAL5 vector-containing strains were
tested and the levels of NUDF were decreased to levels observed in the
nudC3 parental strain (Fig. 5, lanes 7 and 8). The lower
band is nonspecific (dot). Therefore, the results of this
experiment show that c15 expression can rescue the expression of NUDF
at the restrictive temperature.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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The nudC3 allele of nudC, like other temperature-sensitive nud mutants (nudA, nudF, and nudG) grows very slowly at the restrictive temperature (12, 25, 26). The nuclei of these various nud mutants divide normally; however, nuclear migration through the germ tube is arrested. Biochemical studies have shown that the nudC3 mutation affects nuclear migration and growth rate by causing a decrease in the intracellular levels of NUDF, a protein that is essential for nuclear migration in A. nidulans (26). Mutations in the nudF gene that cause a reduction in NUDF protein level also inhibit nuclear movement. Previous work has shown that the effect of the nudC3 mutation on NUDF levels is posttranscriptional, and that NUDC and NUDF are not in complex with each other (26). Recently, a mutation in the A. nidulans cytoplasmic dynein heavy chain that acts as a bypass suppressor of the nudF deletion has been discovered (D. A. Willins and N. R. Morris, unpublished data). This finding suggests that NUDF may serve as an upstream regulator of dynein/dynactin function. In A. nidulans, cytoplasmic dynein is the main motor for nuclear migration, and evidence derived from characterization of the nudC3 mutant phenotype indicates that the nudC gene product probably acts as an upstream modulator of dynein function. Interestingly, the Drosophila nudC gene shows a genetic interaction with the Drosophila Glued gene, which encodes a component of dynactin (J. Cunniff, R. Warrior, Y. Chiu, and N. R. Morris, unpublished data). As a result of these genetic studies, a hierarchy can be established in which NUDC can be placed upstream of NUDF, and both NUDC and NUDF can be placed upstream of dynein and dynactin. Whether or not the NUDC protein directly affects cytoplasmic dynein or its functional partner, dynactin, is not known (19, 34).
The nudF gene encodes a 49-kDa WD-40 protein with a putative coiled-coil domain, similar to the ß-subunit of heterotrimeric G proteins (26, 35, 36). However, NUDF most closely resembles (42% identity) the human lissencephaly-1 gene (LIS-1) (27). Lissencephaly is a neuronal migration disease characterized by the inability of neurons to migrate to their proper positions in the cerebral cortex (37). There is evidence to suggest that neurons may migrate by first extending a long process through which the nucleus and associated organelles then move to establish a new position for the cell body (38, 39). These observations potentially link the genes and proteins involved in centrosomal positioning and nuclear migration with those involved in neuronal migration and brain development. LIS-1 is 99% identical to the 45-kDa regulatory subunit of bovine brain platelet-activating factor acetylhydrolase (PAFAH) (40). The PAFAH enzyme inactivates platelet-activating factor, a lipid second messenger, by removing the acetyl group at the sn-2 position (41, 42). The structural similarities that exist between PAFAH, LIS-1, and NUDF suggest that these proteins may be involved in PAFAH-associated functions. Thus, NUDC may play a role in regulating the activity or the targeting of the PAFAH complex in mammalian cells.
Centrosome orientation in T cells appears to be mediated by a force on centrosomal MTs just as nuclear movement is mediated by a force on SPB MTs (8, 11). We propose that c15 may play a role in the centrosome reorientation that occurs when a T cell meets an antigen-presenting cell or antigen-bearing target cell (3, 43, 44). In this way, the Golgi apparatus and its associated secretory vesicles are reoriented, allowing transport of vesicles along the centrosomal MTs toward the site of contact (3, 44). Although maximal c15 gene induction in T cells by PRL occurs at the G1/S transition (29), a constitutive level of c15 protein already exists (data not shown), which may participate in the rapid centrosome MTOC reorganization process. A similar kinetics of c15 induction was observed in IL-3-stimulated premyeloid cells which suggests that c15 induction by cytokines is part of an activation response (29). Furthermore, the c15 gene product could be involved in other cytoplasmic dynein-mediated functions other than moving centrosome and nuclei, e.g. vesicle migration or mitosis (45, 46), which occur later in the cell cycle as suggested by the kinetics of c15 induction in PRL-stimulated T cells (29). This additional functional complexity is also suggested by the large difference in size between the 45-kDa c15 protein and the 22-kDa NUDC protein. The size difference is due to a much larger amino-terminal domain in the rat c15 protein, which contains basic and acidic motifs that may be involved in protein-protein interactions.
In summary, we have identified a mammalian (rat) gene from PRL-stimulated T cells, c15, that when expressed in A. nidulans nudC3 mutants can functionally rescue the nuclear movement defect. The fact that the rat c15 protein can replace the nuclear movement function of the A. nidulans nudC gene suggests that c15 and NUDC have similar functions. Because of the structural and functional similarities between c15 and NUDC, c15 is likely a mammalian homolog of nudC, and we have therefore named it RnudC. Additionally, c15/RNUDC is widely represented throughout eukaryotes, as immunoreactive c15/RNUDC-like proteins have been detected in Drosophila (J. Cunniff and R. Warrior, personal communication), monkey, and man (data not shown).
Future studies will address the localization of c15/RNUDC in the cell and its association with other proteins. We are particularly interested in determining whether c15/RNUDC localizes to the centrosome MTOC or to the cortex in PRL-stimulated Nb2 T cells. These studies will help determine the role of c15/RNUDC in the events leading to T cell activation, proliferation, and differentiation.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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Complementation Vectors
The 100A plasmid, containing the entire rat c15 ORF
(29), was used as a template to generate a 1.0-kb DNA fragment by PCR
using Pfu polymerase (Statagene, La Jolla, CA). The upstream
primer contained a KpnI linker (lowercase):
5'-gcgggtaccgATGGGAGGGGAGCAG-3' (start codon
underlined), and the downstream primer contained an
XbaI linker (lowercase):
5'-gcgtctagaCTAGTTGAATTTGGC-3' (stop codon
underlined). The KpnI/XbaI-digested
PCR product, encoding c15, was cloned downstream of the alcA
promoter in the pAL5 vector (32) and confirmed by sequencing. The c15
vector and pAL5 vector were used to transform A. nidulans
containing the nudC3 mutation.
A. nidulans Transformation
Conidia (1 x 109) were inoculated into 50 ml
YAG without agar supplemented with UU. The conidia were allowed to
germinate for about 5.5 h, or until the emerging germ tube was
visible, at 32 C with shaking. The germinated conidia were harvested by
centrifugation, resuspended in 40 ml lytic mix [20 ml Solution A (0.1
M citric acid, 0.8 M
(NH4)2SO4 pH 5.8 with KOH pellets),
20 ml Solution B (1% yeast extract, 2% sucrose, 40 mM
glucose, trace elements, vitamins), 10 mM
MgSO4, 200 mg BSA (Sigma, St. Louis, MO), 100 mg Novozyme
234 (Sigma), 125 µl glucuronidase (Sigma)], and the cell wall was
digested for 2 h at 32 C with shaking. The protoplasts were washed
two times in Solution C (50 mM citric acid, pH 6.0, 0.4
M (NH4)2SO4, 1%
sucrose) and were resuspended in 1 ml Solution E (0.6 M
KCl, 100 mM CaCl2, 10 mM Tris-HCl,
pH 7.5). Protoplasts (100 µl) were added to 6 µg plasmid DNA,
followed by the addition of 50 µl Solution D (25% polyethylene
glycol 8000, 100 mM CaCl2, 0.6 M
KCl, 10 mM Tris-HCl, pH 7.5), and incubated on ice for 20
min. Next, 1 ml Solution D was added to the protoplasts and allowed to
incubate at room temperature for 30 min. Aliquots (200 µl) of the
protoplast mixture were plated in 3 ml 45 C sucrose top agar (0.5%
yeast extract, 20 mM glucose, 1 M sucrose,
trace elements, vitamins, 1% agar) onto sucrose plates (0.5% yeast
extract, 20 mM glucose, 0.2 M sucrose, trace
elements, vitamins, 1.5% agar), and incubated at 32 C for 3–4
days.
Complementation of nudC3 Mutants with c15
To test for complementation, conidia containing either the c15
or pAL5 construct were streaked on either MAG or glycerol minimal media
lacking UU at 32 C or 42 C. Independently isolated transformants of
each of the two constructs (c15 and pAL5) were plated, and colonies
were allowed to grow for 2 days before analysis. Conidia from the
transformants were isolated and retested for complementation five
times.
Protein Preparation and Western Blot Analysis
To prepare total cellular proteins, 5 x 108
conidia were inoculated into 50 ml supplemented minimal media
containing 10 ml/liter glycerol and incubated for 42 h at 42 C
with shaking. The mycelia were harvested by centrifugation, washed in
ice-cold H2O, collected by filtration through cheesecloth,
and pressed dry. The mycelia were ground in a Tenbroek homogenizer
(Fisher Scientific, Pittsburgh, PA) in 1–2 ml extraction buffer [50
mM Tris-HCl, pH 7.4, 200 mM NaCl, 5
mM EDTA, 5 mM benzamidine, 1 mM
phenylmethylsulfonyl fluoride, 10 µg/ml each of soybean trypsin
inhibitor, aprotinin, leupeptin,
N-tosyl-L-phenylalanine chloromethyl ketone
(Sigma)], and insoluble proteins were removed by centrifugation at
14,000 rpm for 5 min at 4 C. Protein concentration was determined by
Bradford assay (Bio-Rad, Richmond, CA) using BSA as a standard. The
cellular proteins were stored at -20 C.
For Western blotting, 50 µg total cellular proteins were analyzed by 12% SDS-PAGE and transferred to Immobilon P (Millipore, Bedford, MA) or nitrocellulose (Bio-Rad) as previously described (29). To determine c15 expression, the blot was blocked with 5% nonfat milk, 0.2% Tween 20 (Sigma) in Tris-buffered saline, and affinity-purified rabbit anti-c15-C antibodies (S. M. Morris, in preparation) were applied in a 1:500 dilution, followed by the addition of donkey anti-rabbit IgG antibodies conjugated to horseradish peroxidase (Amersham, Arlington Heights, IL) in a 1:2000 dilution. c15 proteins were detected with the enhanced chemiluminescence system as suggested by the manufacturer (Amersham). To detect NUDF, affinity-purified rabbit anti-NUDF antibodies were used at a 1:100 dilution, followed by goat anti-rabbit IgG antibodies coupled to alkaline phosphatase in a 1:2000 dilution, and developed with 5-bromo-4-chloro-3-indoyl phospate p-toluidine salt and p-nitro blue tetrazolium chloride (26).
DAPI Staining of Nuclei
To stain the nuclei of the developing germlings, 5 x
106 conidia were inoculated into Petri dishes containing
sterile coverslips and 25 ml supplemented liquid minimal media with 10
ml/liter glycerol and were incubated at either 32 C or 42 C for 18
h. The coverslips with the attached germlings were rinsed in
H2O and placed in methanol at -20 C for 10 min, rinsed
well with H2O, and placed in acetone at -20 C for 10 min.
The coverslips were rinsed again and placed in a 50 ng/ml DAPI (Sigma)
solution for 10 min. After a final rinse, the coverslips were mounted
in ProLong Antifade (Molecular Probes, Inc., Eugene, OR) and viewed at
1000x using the Zeiss Axiophot system (Zeiss, Jena, Germany) (Baylor
Integrated Microscopy Core, Baylor College of Medicine, Houston,
TX).
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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This work was supported by grants from the American Cancer Society (BE-49J) (to L.-y. Y.-L.) and The Linda and Ronald Finger Lupus Research Center (to S. M. M.)
Received for publication September 9, 1996. Revision received November 19, 1996. Accepted for publication November 21, 1996.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONRESULTSDISCUSSIONMATERIALS AND METHODSREFERENCES |
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