Assignment of disulfide bonds in gp64, a putative cell-cell adhesion protein of Polysphondylium pallidum. Presence of Sushi domains in the cellular slime mold protein.

The 64-kDa membrane-bound glycoprotein of the cellular slime mold Polysphondylium pallidum (referred to as gp64), seems to be implicated in cell-cell adhesion. Previously we have isolated a full-length gp64 cDNA, determined its nucleotide sequence, and found that all cysteine residues in the protein are involved in the formation of disulfide bonds. The disulfide arrangement of the 36 cysteines in gp64 was established by analysis of proteolytically cleaved protein and sequence analysis of cystine-containing fragments. Since gp64 has 36 Cys residues, 18 disulfide bonds must exist and the positions of 15 of them were determined. The 15 disulfide bonds in gp64 constitute five characteristic, so-called Sushi domains. In a Sushi domain, the first Cys in a sequence is connected to the third one and the second Cys to the fourth one. This is the first report describing the presence of Sushi domains in a cellular slime mold protein. From these data, gp64 appears to be distinct from all other previously described cell-adhesion proteins.