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MSJ-1, a Mouse Testis-Specific DnaJ Protein, Is Highly Expressed in Haploid Male Germ Cells and Interacts with the Testis-Specific Heat Shock Protein Hsp70-2¹

^a Department of Biology, University of Milano, 20133 Milano, Italy ^b Department of Biotechnology and Biosciences, University of Milano-Bicocca, 20126 Milano, Italy

ABSTRACT

The MSJ-1 gene encodes a murine DnaJ homologue that is expressed specifically in adult testis. DnaJ proteins act as cochaperones of Hsp70 proteins in promoting diverse cellular functions. In this study we used recombinant MSJ-1 proteins to produce MSJ-1 antiserum and to carry out in vitro binding assays. In a wide immunoscreening of mouse tissues, affinity-purified MSJ-1 antibodies recognize a unique protein of 30 kDa in male germ cells only. MSJ-1 is able to interact with the testis-specific Hsp70-2 protein and can be coimmunoprecipitated with Hsp70-2 from spermatogenic cells; binding of these two chaperones is consistent with the presence of a third component, which is so far unknown. MSJ-1 is weakly detected in early round spermatids, and its protein content increases in cytodifferentiating spermatids where it colocalizes with the developing acrosome and their postnuclear region. Hsp70-2, which is known to be highly expressed in meiotic cells, shows a subcellular localization in late differentiating spermatids that overlaps that of MSJ-1. MSJ-1 is also maintained in testicular and epididymal spermatozoa, where it sharply demarcates into two distinct cell areas; the outer surface of the acrosomal vesicle, and the centrosomal area. On the whole, our findings are consistent with a role for MSJ-1 in acrosome formation and centrosome adjustment during spermatid development, whereas its presence in mature spermatozoa suggests a special function during fertilization, shortly afterward, or both.

FOOTNOTES

First decision: 9 January 2001.

¹ This work was supported by the Cofin98 grant from MURST, Rome, to G.B.

² Correspondence: Giovanna Berruti, Department of Biology, University of Milan, Via Celoria 26, 20133 Milano, Italy. FAX: 39 02 26604361; giovanna.berruti{at}unimi.it

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