Conantokin-G precursor and its role in γ-carboxylation by a vitamin K-dependent carboxylase from a Conus snail

Conantokin-G isolated from the marine snail Conus geographus is a 17-amino acid gamma-carboxyglutamate (Gla)-containing peptide that inhibits the N-methyl-D-aspartate receptor, We describe the cloning and sequence of conantokin-G cDNA and the possible role of the propeptide sequence, The cDNA encodes a 100-amino acid peptide, The N-terminal 80 amino acids constitute the prepro-sequence, and the mature peptide is derived from the remaining C-terminal residues after proteolysis, C-terminal amidation, and a unique posttranslational modification, gamma-carboxylation of glutamate residues to Gla, Mature conantokin-G peptide containing 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 propeptide we investigated a potential role for the propeptide sequences in gamma-carboxylation. Propeptide segment -20 to -1 covalently linked to E.Con-G or the synthetic pentapeptide FLEEL increased their apparent affinities 2 orders of magnitude. These substrates are not efficiently carboxylated by the bovine microsomal gamma-glutamyl carboxylase, suggesting differences in specificities between the Conus and the mammalian enzyme. However, the role of propeptide in enhancing the efficiency of carboxylation is maintained.