Characterization of a SEPT9 interacting protein, SEPT14, a novel testis-specific septin

Septins are a highly conserved family of GTP-binding cytoskeletal proteins implicated in multiple cellular functions, including membrane transport, apoptosis, cell polarity, cell cycle regulation, cytokinesis, and oncogenesis. Here we describe the characterization of a novel interacting partner of the septin family, initially cloned from a human testis expression library following yeast two-hybrid isolation to identify SEPT9 binding partners. Upon further genomic characterization and bioinformatics analyses it was determined that this novel septin-interacting partner was also a new member of the mammalian septin family, named SEPT14. SEPT14 maps to 7p11.2 in humans and includes a conserved GTPase domain and a predicted carboxy-terminus coiled-coil domain characteristic of other septins. Three potential translational start methionines were identified by 5' RACE-PCR encoding proteins of 432-, 427-, and 425-residue peptides, respectively. SEPT14 shares closest homology to SEPT10, a human dendritic septin, and limited homology to SEPT9 isoforms. SEPT14 colocalized with SEPT9 when coexpressed in cell lines, and epitope-tagged forms of these proteins coimmunoprecipitated. Moreover, SEPT14 was coimmunoprecipitated from rat testes using SEPT9 antibodies, and yeast two-hybrid analysis suggested SEPT14 interactions with nine additional septins. Multitissue Northern blotting showed testis-specific expression of a single 5.0-kb SEPT14 transcript. RT-PCR analysis revealed that SEPT14 was not detectable in normal or cancerous ovarian, breast, prostate, bladder, or kidney cell lines and was only faintly detected in fetal liver, tonsil, and thymus samples. Interestingly, SEPT14 was expressed in testis but not testicular cancer cell lines by RT-PCR, suggesting that further investigation of SEPT14 as a testis-specific tumor suppressor is necessary.