δ/ω-Plectoxin-Pt1a: an excitatory spider toxin with actions on both Ca(2+) and Na(+) channels

The venom of spider Plectreurys tristis contains a variety of peptide toxins that selectively target neuronal ion channels. O-palmitoylation of a threonine or serine residue, along with a characteristic and highly constrained disulfide bond structure, are hallmarks of a family of toxins found in this venom. Here, we report the isolation and characterization of a new toxin, δ/ω-plectoxin-Pt1a, from this spider venom. It is a 40 amino acid peptide containing an O-palmitoylated Ser-39. Analysis of δ/ω-plectoxin-Pt1a cDNA reveals a small precursor containing a secretion signal sequence, a 14 amino acid N-terminal propeptide, and a C-terminal amidation signal. The biological activity of δ/ω-plectoxin-Pt1a is also unique. It preferentially blocks a subset of Ca(2+) channels that is apparently not required for neurotransmitter release; decreases threshold for Na(+) channel activation; and slows Na(+) channel inactivation. As δ/ω-plectoxin-Pt1a enhances synaptic transmission by prolonging presynaptic release of neurotransmitter, its effects on Na(+) and Ca(2+) channels may act synergistically to sustain the terminal excitability.