RECOMBINANT PRESPORE-SPECIFIC ANTIGEN FROM DICTYOSTELIUM-DISCOIDEUM IS A BETA-SHEET GLYCOPROTEIN WITH A SPACER PEPTIDE MODIFIED BY O-LINKEDN-ACETYLGLUCOSAMINE

Prespore-specific antigen (PsA) is a putative cell-adhesion molecule o f the cellular slime mould Dictyostelium discoideum, which has a simil ar molecular architecture to several mammalian cell-surface proteins. It has an N-terminal globular domain presented to the extracellular en vironment on an O-glycosylated stem (glycopeptide) that is attached to the cell membrane through a glycosyl-PtdIns anchor. The sequence of P sA suggests that PsA may belong to a new family of cell-surface molecu les and here we present information on the structure of the N-terminal globular domain and determine the reducing-terminal linkage of the O- glycosylation. To obtain a sufficient amount of pure protein, a secret ed recombinant form of PsA (rPsA), was expressed in D. discoideum and characterised. H-1-NMR spectra of rPsA contained features consistent w ith a high degree of P-sheet in the N-terminal globular domain, a feat ure commonly observed in eel-adhesion proteins. Solid-phase Edman degr adation of the glycopeptide of rPsA indicated that 14 of the 15 threon ines and serines in the spacer region were glycosylated. The chemical structures of the O-glycosylations were determined to be single N-acet ylglucosamine residues.