Molecular cloning and sequencing of Calnexin-t: An abundant male germ cell-specific calcium-binding protein of the endoplasmic reticulum

A mouse testis cDNA expression library was screened using a monoclonal antibody (1C9) that recognized an abundant testis-specific 101-kDa endoplasmic reticulum-associated protein. The screening resulted in the isolation of a 2.3-kilobase cDNA clone (A2/6). The sequence encoded 611 amino acids with a calculated mass of 69,454 Da, that was 60% similar to mouse calnexin. A high affinity calcium binding domain, present in both calnexin and calreticulin, and one transmembrane domain similar to that of calnexin were found in the A2/6 protein domain. Northern blot analysis of total RNA from seven different tissues showed hybridization only to testis RNA. Southern blot analysis indicated that A2/6 was a single copy gene. The calculated molecular mass for A2/6 was unexpectedly lower than the 101-kDa protein recognized by 1C9 on Western blot analysis of total testis protein. However, Escherichia coli and in vitro translation products of A2/6 cDNA yielded a similar 100-kDa protein. Finally, using the recombinant protein, calcium binding activity was detected by a ⁴⁵Ca²⁺ overlay assay. These results suggest that spermatogenic cell endoplasmic reticulum has a unique calcium binding protein, calnexin-t, which appears to be a calnexin variant.