SPERM PROTEIN (SP50) BINDS TO ACROSOME AND PLASMA-MEMBRANES IN A CA2-DEPENDENT MANNER - POSSIBLE ROLE IN ACROSOME REACTION()

Annexins are a family of Ca2+-binding proteins involved in the exocyto tic process. The presence and the role of annexins in mammalian sperma tozoa have not been well established. Two annexin-like proteins were o btained from guinea pig testis, a doublet of Mr 31-33 kD (p31/33) and a protein of Mr 50 kD (p50). Both proteins were able to bind to erythr ocyte ghosts in a Ca2+ dependent fashion. Polyclonal antibodies agains t p31/33 reacted with two major proteins, Mrs 50 kD (sp50) and 42 kD ( sp42), from mature and immature guinea pig spermatozoa. p50 and sp50 a re likely the native proteins from testis and spermatozoa, respectivel y, and they are seemingly related. By immunofluorescence, sp50 was onl y found in the apical acrosome region of immature and capacitated and noncapacitated spermatozoa, and its location was intracellular. In spe rmatozoa undergoing acrosome reaction, sp50 was detected in the whole acrosome, while in spermatozoa that had undergone acrosome reaction sp 50 was not detected. However, in the protein pattern of acrosome react ion vesicles, anti-p31/33 antibody revealed diffuse bands of Mr 35-38 kD. sp50 was able to bind to plasma membrane fragments and acrosome ou ter membrane from demembranated sperm in a Ca2+-dependent fashion. The presence of sp50 in the acrosome region, its distribution throughout the acrosome membrane just before the acrosome reaction, and its abili ty to bind both plasma and outer acrosome membranes in a Ca2+-dependen t manner suggest that sp50 may par ticipate in the acrosome reaction m echanism in guinea pig spermatozoa. (C) 1996 Wiley-Liss, Inc.