Identification of heterotrimeric G proteins in human sperm tail membranes

Heterotrimeric G proteins play important roles as signal transducing components in various mammalian sperm functions. We were interested in the distribution of G proteins in human sperm tails. Prior to membrane preparation, spermatozoa were separated from contaminating cells which are frequently present in human ejaculates. Enriched human sperm tail membranes were generated by using hypoosmotic swelling and homogenization procedures. Antisera against synthetic peptides were used to identify G proteins in immunoblots. AS 8, an antiserum directed against an amino acid sequence that is found in most G protein alpha-subunits, and A 86, which detects all known pertussis toxin-sensitive alpha-subunits, reacted specifically with a 40-kDa protein. Antisera against individual G protein alpha-subunits failed to detect any specific antigens in enriched tail membranes. AS 36, recognizing the beta 2-subunit of G proteins, identified a 35-kDa protein in sperm tail membranes. Antisera against the 36-kDa beta 1-subunit did not detect any relevant proteins in the membrane fraction. Neither G protein alpha-subunits nor G protein beta-subunits were found in the cytosol. ADP ribosylation of spermatozoal membrane or cytosolic proteins revealed no pertussis toxin-sensitive alpha-subunits. However, membrane preparations of nonpurified human spermatozoa contained alpha i2 subunits, as shown immunologically and by ADP ribosylation; they most probably derived from somatic cells which are frequently present in human ejaculates. Our results stress the fact that spermatozoa need to be purified before sperm membrane preparation to avoid misinterpretations caused by contaminating cells. Furthermore, we suggest that G proteins in membranes of human sperm tails belong to a novel subtype of G protein alpha-subunits; the putative beta-subunit was identified as a beta 2-subunit.