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The Rat 78,000 Dalton Glucose-Regulated Protein (GRP78) as a Precursor for the Rat Steroidogenesis-Activator Polypeptide (SAP): The SAP Coding Sequence is Homologous with the Terminal End of GRP78

Department of Biochemistry Los Angeles, California 90033
Department of Physiology and Biophysics Los Angeles, California 90033
University of Southern California School of Medicine, Norris Cancer Research Institute Los Angeles, California 90033
Department of Biochemistry, School of Medicine and Biochemical Sciences, State University of New York Buffalo, New York 14214

Address requests for reprints to: Dr. Amy S. Lee, University of Southern California School of Medicine, Department of Biochemistry, Los Angeles, California 90033.

Abstract

Based on the striking sequence identity between the amino acid sequence of rat steroidogenesis-activator polypeptide (SAP) and the carboxyl terminus of the 78,000 dalton glucose-regulated protein (GRP78), the precursor-product relationship between GRP78 and SAP was investigated in Leydig cells. Immunoblot analysis with peptide antibodies specific for GRP78 and SAP showed that the putative SAP precursor is also immunoreactive with the anti-GRP78 antibody. Genomic blot hybridizations further revealed that GRP78 is neither rearranged nor amplified in the H-540 Leydig cell tumor, the original source for SAP. Further, there appears to be a single copy of the SAP coding sequence within the rat genome. This sequence resides within the last exon of GRP78. Our observations support the hypothesis that, in steroidogenic cells, SAP is likely to be derived from posttranslational processing of a very minor fraction of GRP78.

INTRODUCTION: The 78,000 dalton glucose-regulated protein (GRP78) is a stress-inducible protein which is localized in the endoplasmic reticulum (ER) (1). This protein has also been identified as the heavy chain binding protein found in B lymphocytes in association with unassembled immunoglobulin heavy chains (2, 3).

FOOTNOTES

This research is supported in part by the Health and Human Services Grant R37 CA-27607 (to A.S.L.) USPHS Grant DK-18141 (to R.C.P.), and USPHS Grant HD-15999 (to D.W.W.).

^* Recipient of Research Career Development Award HD-00613.

Received for publication September 6, 1989. Revision received September 26, 1989. Accepted for publication September 26, 1989.

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