Alpha-conopeptides specifically expressed in the salivary gland of Conus pulicarius

Abstract

To date, studies conducted on cone snail venoms have attributed the origins of this complex mixture of neuroactive peptides entirely to gene expression by the secretory cells lining the lumen of the venom duct. However, specialized tissues such as the salivary glands also secrete their contents into the anterior gut and could potentially contribute some venom components injected into target animals; evidence supporting this possibility is reported here. Sequence analysis of a cDNA library created from a salivary gland of Conus pulicarius revealed the expression of two transcripts whose predicted gene products, after post-translational processing, strikingly resemble mature conopeptides belonging to the alpha-conotoxin family. These two transcripts, like alpha-conotoxin transcripts, putatively encode mature peptides containing the conserved A-superfamily cysteine pattern (CC-C-C) but the highly conserved A-superfamily signal sequences were not present. Analysis of A-superfamily members expressed in the venom duct of the same C. pulicarius specimens revealed three putative alpha-conotoxin sequences; the salivary gland transcripts were not found in the venom duct cDNA library, suggesting that these alpha-conotoxins are salivary gland specific. Therefore, expression of conotoxin-like gene products by the salivary gland could potentially add to the complexity of Conus venoms.

Fig. 1

A picture of the shell of one of the actual Conus pulicarius specimens collected from Pago Bay, Guam. C. pulicarius is a vermivorous cone snail that inhabits shallow coastal waters throughout the Indo-Pacific.

Fig. 2

Conus pulicarius salivary gland toxin precursors contain α4/7-conotoxin-like Cys patterns. Identical residues between the sequences are enclosed within boxes. The ends of the predicted secretion signals and proteolytic cleavage sites are delineated by arrows. Both of these precursors are predicted to yield mature toxins resembling members of the well-characterized alpha4/7 conopeptide subfamily.

Fig. 3

Comparison of the three toxin precursors identified from a Conus pulicarius venom duct from Pago Bay, Guam. Identical residues between the sequences are enclosed within boxes. The ends of the predicted secretion signals and proteolytic cleavage sites are delineated by arrows. The cysteine residues are shaded in black. Each of these precursors is predicted to yield mature toxins resembling members of the well-characterized alpha4/7 conopeptide subfamily.

Fig. 4

Comparison of the two Conus pulicarius salivary gland toxin precursors with the standard A-superfamily precursors. PuSG1.1 and PuSG1.2 toxin precursors are compared to the sequences of α-MII, and three toxin precursors (Pu1.4, Pu1.5, and Pu1.66) from the venom duct of the same snail. Amino acid residues within the venom duct sequences that are identical to α-MII are shaded in grey; amino acid residues within the salivary gland precursors that are identical to α-MII are delineated by boxes. The cysteine residues are shaded in black.