Phosphatidylethanolamine binding protein is an abundant secretory product of haploid testicular germ cells in the rat

Abstract

An abundant cellular and secretory product of isolated seminiferous tubules from adult rats was identified as having an apparent molecular weight of ∼24 000 and a pI of 5.3 on autoradiographs of two-dimensional polyacrylamide gels. A protein with identical migration characteristics was identified as a major secretory product of isolated round spermatids. Microsequencing revealed that the protein had homology to phosphatidylethanolamine binding protein (PEBP) identified in rat brain. Primers were used in conjunction with RTPCR to amplify a partial cDNA which was used to probe a rat testis library to obtain full lenght clones. On Northern blots, PEBP mRNA was abundant in adult rat testis and epididymis and fractions enriched in germ cells but was very low/absent from fetal or immature rat testis or adult rat Sertoli cells. In situ hybridisation identified that abundant mRNA was first detectable in pachytene spermatocytes at stage VII and thereafter at particularly high levels in round and elongating spermatids until step 14. Proteins with significant sequence homology to the rat testis PEBP have been identified previously in mouse testis and epididymis, in rat germ cell cultures and coating the surface of mature rat sperm. Differences in the timing of expression of the PEBP mRNA (first expressed in pachytene spermatocytes) and secretion of the PEBP protein (not a major secretory product until round spermatids) is consistent with PEBP mRNA undergoing delayed translation. The role(s) of secreted lipid binding proteins in spermatogenesis are discussed.