Nicotinic acetylcholine receptor subunits and associated proteins in human sperm

We demonstrated previously the involvement of a nicotinic acetylcholine receptor containing an alpha 7 subunit in the human sperm acrosome reaction ( a modified exocytotic event essential to fertilization). Here we report the presence in human sperm of alpha 7, alpha 9, alpha 3, alpha 5, and beta 4 nicotinic acetylcholine receptor subunits and the following proteins known to be associated with the receptor in the somatic cell: rapsyn and the tyrosine kinases c- SRC and FYN. The alpha 7 subunit appears to exist as a homomer in the posterior post- acrosomal and neck regions of sperm and is probably linked to the cytoskeleton via rapsyn. The alpha 3, alpha 5, and beta 4 subunits are present in the sperm flagellar mid- piece of sperm and possibly exist as alpha 3 alpha 5 beta 4 and/ or alpha 3 beta 4 channels. The alpha 9 subunit is present in the sperm mid- piece. We detected the FYN and c- SRC tyrosine kinases in the flagellar mid- piece region. Both co- precipitated only with the nicotinic acetylcholine receptor beta 4 subunit. Immunolocalization with a C- terminal SRC kinase antibody, which recognizes several members of SRC kinase family, detected a SRC kinase co- localized with the alpha 7 subunit in the neck region of sperm. Immunoprecipitation studies with that antibody demonstrated that the alpha 7 subunit is associated with a SRC kinase. Antagonists of tyrosine phosphorylation inhibited the acetylcholine- initiated acrosome reaction, suggesting the involvement of a SRC kinase in the acrosome reaction.