Protein dynamics in sperm membranes: implications for sperm function during gamete interaction

A number of mammalian sperm plasma membrane antigens have been implicated as playing a functional role in sperm-egg interaction, by virtue of the fact that antibodies against these antigens interfere with fertilization. Two such mouse sperm plasma membrane antigens are M42, a 200/220 kD glycoprotein doublet, and M5, a 150-160 kD glycoprotein. We show that both of these antigens are concentrated on the posterior region of caudal epididymal and capacitated mouse sperm heads and are relatively diffusible, as determined by fluorescence recovery after photobleaching measurements (D = 3-8 x 10(-9) cm2/s with approximately 23% diffusing). Crosslinking of these antigens with bivalent antibodies causes them to redistribute into the anterior region (acrosomal crescent) of the sperm head. In contrast, we describe a third antigen, P220, which is also localized to the posterior region of the sperm head on caudal epididymal sperm but which exhibits very little diffusion and does not redistribute upon crosslinking. Bivalent anti-M42 blocks the ZP3-induced acrosome reaction. We have found that monovalent Fab fragments of anti-M42 do not block the ZP3-induced acrosome reaction, but that inhibition is restored by addition of a second antibody which crosslinks the Fabs. Thus, crosslinking is required for both inhibition of the acrosome reaction and redistribution. This suggests that redistribution of antigen away from the posterior region of the head may be part of the mechanism of inhibition of the ZP3-induced acrosome reaction.