PHOSPHATIDYLETHANOLAMINE BINDING-PROTEIN IS AN ABUNDANT SECRETORY PRODUCT OF HAPLOID TESTICULAR GERM-CELLS IN THE RAT

An abundant cellular and secretory product of isolated seminiferous tu bules from adult rats was identified as having an apparent molecular w eight of similar to 24 000 and a pI of 5.3 on autoradiographs of two-d imensional polyacrylamide gels. A protein with identical migration cha racteristics was identified as a major secretory product of isolated r ound spermatids. Microsequencing revealed that the protein had homolog y to phosphatidylethanolamine binding protein (PEBP) identified in rat brain. Primers were used in conjunction with RTPCR to amplify a parti al cDNA which was used to probe a rat testis library to obtain full le ngth clones. On Northern blots, PEBP mRNA was abundant in adult rat te stis and epididymis and fractions enriched in germ cells but was very low/absent from fetal or immature rat testis or adult rat Sertoli cell s. In situ hybridisation identified that abundant mRNA was first detec table in pachytene spermatocytes at stage VII and thereafter at partic ularly high levels in round and elongating spermatids until step 14. P roteins with significant sequence homology to the rat testis PEBP have been identified previously in mouse testis and epididymis, in rat ger m cell cultures and coating the surface of mature rat sperm. Differenc es in the timing of expression of the PEBP mRNA (first expressed in pa chytene 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.