Mu-conotoxins block voltage-gated sodium channels (Nav). This toxin blocks rNav1.5/SCN5A (IC(50) is 1.3 uM), rNav1.6/SCN8A (IC(50) is 160 nM), rNav1.7/SCN9A (IC(50) is 1.3 uM), rNav1.1/SCN1A (K(d) is 3.8 nM), rNav1.2/SCN2A (K(d) is 1.3 nM), rNav1.4/SCN4A (K(d) is 0.22 nM), rNav1.6/SCN8A (K(d) is 69 nM), and rNav1.7/SCN9A (K(d) is 260 nM). This toxin is very potent but weakly discriminating among sodium channels. The block of these channels is modified when beta-subunits are coexpressed with alpha subunits. Hence, blocks of channels containing beta-1 and beta-3 subunits are more potent (compared to channels without beta subunits), whereas blocks of channels containing beta-2 and beta-4 subunits are less potent (compared to channels without beta subunits). Expressed by the venom duct. The cysteine framework is III (CC-C-C-CC). Classified in the M-5 branch, since 5 residues stand between the fourth and the fifth cysteine residues. SmIIIA' is a putative isoform where the N-terminal AA is missing. All peptide bonds in this toxin adopt the trans conformation. Does not show activity on rNav1.8/SCN10A (IC(50) is >10 uM). Belongs to the conotoxin M superfamily. Mu-conotoxin SmIIIA entry