403 IDENTIFICATION AND CHARACTERIZATION OF A NOVEL MOUSE AND HUMAN MOPT GENE CONTAINING MORN MOTIF PROTEIN IN TESTIS

By subtraction screening methods, we identified a novel mouse and its counterpart, human MOPT gene. The mouse and human MOT gene is localized on mouse chromosome 17E3 and human chromosome 2p22, and spans approximately 7 kb. Analysis of the mouse Mopt sequence revealed the existence of an ORF of 240 bp encoding a putative protein of 79aa amino acids. The predicted protein has a theoretical molecular mass of 8.89 kDa and a calculated isoelectric point of 5.82. The protein was unique; it did not show any similarities with other known protein except for Morn motif domain. Real-time reverse transcriptase polymerase chain reaction and Northern blot analysis revealed that mouse Mopt transcripts are highly and specifically expressed in adult testis and skeletal muscle. In situ hybridization and immunohistochemistry studies showed that mouse Mopt transcript and protein was confined mainly to round and elongated spermatids, except for a few individual dispersed spermatocytes, and increases in abundance in subsequent stages. To characterize Mopt functions in spermiogenesis, we examined Mopt protein distribution in late spermiogenesis by using immunogold electron microscopy: Mopt protein first appeared in the proacrosomic vesicles of the early Golgi phase spermatids. In the final step of spermiogenesis, Mopt expression was translocated from the head cap of an elongated spermatid to the nucleus of mature spermatozoa. However, no other testicular cell types, including somatic cells, spermatogenic cells, and residual cytoplasm that ultimately is engulfed as residual bodies into Sertoli cells, were found to bear Mopt-positive staining. This observation suggested that Mopt may play an important role in dynamic regulation of acrosome biogenesis during late spermiogenesis and an as yet uncharacterized role in oocyte activation during capacitation, after fertilization, or both.