DISTRIBUTION AND DYNAMICS OF MOUSE SPERM SURFACE GALACTOSYLTRANSFERASE - IMPLICATIONS FOR MAMMALIAN FERTILIZATION

It has been proposed that a mouse sperm surface beta-1,4-galactosyltransferase functions as a receptor for the zona pellucida during fertilization. In this paper we used two monovalent fluorescent probes specific for galactosyltransferase: a trinitrophenylated derivative of UDP-galactose and rhodaminated alpha-lactalbumin. We found that galactosyltransferase was initially present over the posterior head of acrosome-intact sperm but became progressively localized to the plasma membrane overlying the acrosomal region after it was cross-linked with an anti-galactosyltransferase polyclonal antibody. Labeled mouse sperm that were treated with the calcium ionophore A23187 revealed that galactosyltransferase remained on the posterior head after acrosomal exocytosis. However, if galactosyltransferase was first cross-linked and redistributed with antibody and then acrosome reacted with A23187, all head fluorescence was lost. In addition, although anti-galactosyltransferase antibody induced a surface redistribution, it did not, by itself, lead to the release of acrosin, the endpoint of the acrosome reaction. Finally, using the technique of fluorescence recovery after photobleaching, we found that, in the absence of bivalent antibody, mouse sperm surface galactosyltransferase exhibited 40-50% recovery with a high diffusion coefficient on the anterior head (5-8 x 10(-9) cm(2)/s) approximately 2 times greater than on the posterior head (2-4 x 10(-9) cm(2)/s). When galactosyltransferase was cross-linked and redistributed to the anterior head using the bivalent antibody, the mobile fraction decreased to 20-30% with no significant change in the diffusion coefficient. These data demonstrate that mouse sperm surface galactosyltransferase has many important properties requisite for its function as a receptor for the zona pellucida, but bivalent cross-linking of mouse sperm surface galactosyltransferase is not sufficient to induce the final signal transduction events leading acrosomal exocytosis.