Identification and characterization of new cDNAs encoding outer dense fiber proteins of rat sperm.

A main structure of the mammalian sperm tail is a structure known as the outer dense fibers whose molecular composition as well as their function are still mostly unknown. We report here the isolation and characterization of new cDNAs (odf2) that identifies a highly variable gene locus encoding outer dense fiber proteins. Transcription of odf2 is restricted to testis and more specifically to round spermatids. Transcription starts in step 6 spermatids, which coincides with transcription of the major outer dense fiber protein gene odf1 (Burmester, S., and Hoyer-Fender, S. (1996) Mol. Reprod. Dev. 45, 10-20) and with the formation of the sperm tail. Affinity-purified anti-Odf2 antibodies identified isolated outer dense fibers immunocytochemically and detected at least three protein bands in the molecular mass range of 65,000 to 70,000 Da in total Odf protein preparations. Presence of several protein bands correlates with the presence of several transcripts and the isolation of slightly different cDNA clones, whereas Southern blot hybridization does not indicate the presence of multiple genes. Computer analyses of the structure of the encoded Odf2 protein revealed an overall alpha-helical structure with two regions identical to the dimerization region of the leucine zipper motif.