Sperm adhesion molecules: structure and function.
Fertilisation is a unique event in which the morphologically disparate gametes recognise, bind and fuse with each other. This event follows a highly regulated schedule of biochemical interactions, in which molecules are involved that mediate cell adhesion, signal transduction and the initiation of metabolic pathways. A plethora of molecules has been found on the male gamete and with regard to the different protein structures it is almost impossible to overlook the structures involved. Even more, carbohydrate structures cause an additional diversity with regard to the generation of surface structures. In this communication we try to elucidate the structures of proteins that have been known so far. We have focussed on spermadhesins, the zonadhesin, proacrosin and the PH-20 antigen. The variety of structures and also the common features among them as well as the presence of redundant systems are attributable to the evolutionary force of intraspecific sperm competition. This evolutionary force is assumed to be also responsible for the species selectivity observed in these adhesion molecules, which explains the preferential binding of gametes in a homologous system.