Testis

Novel Ring Finger Protein OIP1 Binds to Conserved Amino Acid Repeats in Sperm Tail Protein ODF1

Abstract

Outer dense fibers (ODF) and fibrous sheath (FS) are unique structures of the mammalian sperm tail. Recently, progress has been made into the molecular cloning of ODF and FS proteins and because of this, questions addressing the morphogenesis, underlying protein network that make up sperm tail structures and their function can now be addressed. Using the N-terminal leucine zipper motif of the major ODF protein ODF1 we had previously isolated interacting proteins ODF2, SPAG4 and SPAG5. We report here a yeast-two hybrid strategy to isolate a novel rat testicular protein, OIP1, that binds to the evolutionary conserved Cys-Gly-Pro repeats in the C-terminus of ODF1. OIP1 is expressed in round spermatids as well as in spermatocytes and several somatic tissues, albeit at a lower level. No expression was detectable in epididymis, heart and smooth muscle. OIP1 protein localizes to the sperm tail, in a pattern expected for an ODF1-interacting protein. OIP1 belongs to the family of RING finger proteins of the H2 subclass. Deletion of the putative RING motif significantly decreased binding to ODF1. Genomics analysis of rat Oip1 and Oip1 homologs indicates that Oip1 is highly conserved. Oip1 is subject to differential splicing and alternative polyadenylation events. Interestingly, Oip1 mRNAs have been reported that lack the exon encoding the putative RING finger.