STRUCTURE AND FUNCTION OF THE N-LINKED CARBOHYDRATE CHAINS OF PIG ZONA-PELLUCIDA GLYCOPROTEINS

The N-linked carbohydrate chain of the PZP3 glycoprotein family of pig zona pellucida is shown by competition assay to possess sperm recepto r activity. Structural analysis reveals that the N-linked chains compr ise neutral and acidic complex-type chains. The tri- or tetra-antennar y fraction in the neutral chains possesses the sperm receptor activity , but the activity is markedly reduced compared with that of PZP3, sug gesting that the protein moiety participates in the maintenance of pro per orientation of the active chain. Two components in the PZP3 family can be separated by reverse-phase HPLC after endo-beta-galactosidase (E beta G) digestion and one of the digests, E beta G-PZP3 alpha, poss esses the sperm receptor activity. E beta G-PZP alpha deprived of O-li nked chains by treatment with alkali retains the activity comparable w ith E beta G-PZP3 alpha, while E beta G-PZP3 alpha deprived of N-linke d chains by N-glycanase digestion loses the activity. Furthermore, com petition assay of the digests of E beta G-PZP3 alpha with lysyl-endope ptidase indicates that the active chain is linked to Asn67 or Asn84 of PZP3 alpha or to both residues. We conclude that sperm-egg binding in pigs is mainly mediated by tri- or tetra-antennary, neutral, complex- type N-linked carbohydrate chain(s) localized in the N-terminal region of PZP3 alpha protein.