CONANTOKIN-G PRECURSOR AND ITS ROLE IN GAMMA-CARBOXYLATION BY A VITAMIN-K-DEPENDENT CARBOXYLASE FROM A CONUS SNAIL

Conantokin-G isolated from the marine snail Conus geographus is a 17-a mino acid gamma-carboxyglutamate (Gla)-containing peptide that inhibit s the N-methyl-D-aspartate receptor, We describe the cloning and seque nce of conantokin-G cDNA and the possible role of the propeptide seque nce, The cDNA encodes a 100-amino acid peptide, The N-terminal 80 amin o acids constitute the prepro-sequence, and the mature peptide is deri ved from the remaining C-terminal residues after proteolysis, C-termin al amidation, and a unique posttranslational modification, gamma-carbo xylation of glutamate residues to Gla, Mature conantokin-G peptide con taining Glu residues (E.Con-G) in place of Gla is a poor substrate for the vitamin K-dependent gamma-glutamyl carboxylase (apparent K-m = 3. 4 mM). Using peptides corresponding to different segments of the prope ptide we investigated a potential role for the propeptide sequences in gamma-carboxylation. Propeptide segment -20 to -1 covalently linked t o E.Con-G or the synthetic pentapeptide FLEEL increased their apparent affinities 2 orders of magnitude. These substrates are not efficientl y carboxylated by the bovine microsomal gamma-glutamyl carboxylase, su ggesting differences in specificities between the Conus and the mammal ian enzyme. However, the role of propeptide in enhancing the efficienc y of carboxylation is maintained.