MSJ-1, a mouse testis-specific DnaJ protein, is highly expressed in haploid male germ cells and interacts with the testis-specific heat shock proteinHsp70-2

The MSJ-1 gene encodes a murine DnaJ homologue that is expressed specifical ly in adult testis. DnaJ proteins act as co-chaperones of Hsp70 proteins in promoting diverse cellular functions. in this study we used recombinant MS J-1 proteins to produce MSJ-1 antiserum and to carry out in vitro binding a ssays. in a wide immunoscreening of mouse tissues, affinity-purified MSJ-1 antibodies recognize a unique protein of 30 kDa in male germ cells only. MS J-1 is able to interact with the testis-specific Hsp70-2 protein and can be coimmunoprecipitated with Hsp70-2 from spermatogenic cells; binding of the se two chaperones is consistent with the presence of a third component, whi ch is so far unknown. MSJ-1 is weakly detected in early round spermatids, a nd its protein content increases in cytodifferentiating spermatids where it colocalizes with the developing acrosome and their postnuclear region. Hsp 70-2, which is known to be highly expressed in meiotic cells, shows a subce llular localization in late differentiating spermatids that overlaps that o f MSJ-1. MSJ-1 is also maintained in testicular and epididymal spermatozoa, where it sharply demarcates into two distinct cell areas; the outer surfac e of the acrosomal vesicle, and the centrosomal area. On the whole, our fin dings are consistent with a role for MSJ-1 in acrosome formation and centro some adjustment during spermatid development, whereas its presence in matur e spermatozoa suggests a special function during fertilization, shortly aft erward, or both.