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

It has been proposed that a mouse sperm surface beta-1,4-galactosyltra nsferase functions as a receptor for the zona pellucida during fertili zation. In this paper we used two monovalent fluorescent probes specif ic for galactosyltransferase: a trinitrophenylated derivative of UDP-g alactose and rhodaminated alpha-lactalbumin. We found that galactosylt ransferase was initially present over the posterior head of acrosome-i ntact 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 we re treated with the calcium ionophore A23187 revealed that galactosylt ransferase remained on the posterior head after acrosomal exocytosis. However, if galactosyltransferase was first cross-linked and redistrib uted with antibody and then acrosome reacted with A23187, all head flu orescence was lost. In addition, although anti-galactosyltransferase a ntibody induced a surface redistribution, it did not, by itself, lead to the release of acrosin, the endpoint of the acrosome reaction. Fina lly, using the technique of fluorescence recovery after photobleaching , we found that, in the absence of bivalent antibody, mouse sperm surf ace galactosyltransferase exhibited 40-50% recovery with a high diffus ion coefficient on the anterior head (5-8 x 10(-9) cm(2)/s) approximat ely 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 decreas ed 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 sur face galactosyltransferase is not sufficient to induce the final signa l transduction events leading acrosomal exocytosis.