THE SOLUTION STRUCTURE OF OMEGA-AGA-IVB, A P-TYPE CALCIUM-CHANNEL ANTAGONIST FROM VENOM OF THE FUNNEL-WEB SPIDER, AGELENOPSIS-APERTA

The 48 amino acid peptides omega-Aga-IVA and omega-Aga-IVB are the fis t agents known to specifically block P-type calcium channels in mammal ian brain, thus complementing the existing suite of pharmacological to ols used for characterizing calcium channels. These peptides provide a new set of probes for studies aimed at elucidating the structural bas is underlying the subtype specificity of calcium channel antagonists. We used 288 NMR-derived constraints in a protocol combining distance g eometry and molecular dynamics employing the program DGII, followed by energy minimization with Discover to derive the three-dimensional str ucture of omega-Aga-IVB. The toxin consists of a well-defined core reg ion, comprising seven solvent-shielded residues and a well-defined tri ple-stranded beta-sheet. Four loop regions have average backbone rms d eviations between 0.38 and 1.31 Angstrom, two of which are well-define d type-II beta-turns. Other structural features include disordered C- and N-termini and several conserved basic amino acids that are cluster ed on one face of the molecule. The reported structure suggests a poss ible surface for interaction with the channel. This surface contains a mino acids that are identical to those of another known P-type calcium channel antagonist, omega-Aga-IVA, and is rich in basic residues that may have a role in binding to the anionic sites in the extracellular regions of the calcium channel.