Spag4, a novel sperm protein, binds outer dense-fiber protein Odf1 and localizes to microtubules of manchette and axoneme

Outer dense fibers are structures unique to the sperm tail. No definite fun ction for these fibers has been found, but they may play a role in motility and provide elastic recoil. Their composition had been described before, b ut only two of the fiber proteins, Odf1 and Odf2, are cloned. We cloned Odf 2 by virtue of its functional and specific interaction with Odf1, which, we show, is mediated by a leucine zipper. Further work demonstrated that the 84-kDa Odf2 protein localizes to both the cortex and the medulla of the fib ers, whereas the 27-kDa Odf1 protein is present only in the medulla. Here w e report the cloning and characterization of a new Odf1-interacting protein , Spag4. Spag4 mRNA is spermatid specific, and the 49-kDa Spag4 protein com plexes specifically with Odf1, but not Odf2, mediated by a leucine zipper. It also self-associates. In contrast to Odf1 and Odf2, Spag4 protein locali zes to two microtubule-containing spermatid structures. Spag4 is detectable in the transient manchette and it is associated with the axoneme in elonga ting spermatids and epididymal sperm. Our data suggest a role for Spag4 in protein localization to two major sperm tail structures. (C) 1999 Academic Press.