δ/ω-Plectoxin-Pt1a: An Excitatory Spider Toxin with Actions on both Ca2+ 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, delta/omega-plectoxin-Pt1a, from this spider venom. It is a 40 amino acid peptide containing an O-palmitoylated Ser-39. Analysis of delta/omega-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 delta/omega-plectoxin-Pt1a is also unique. It preferentially blocks a subset of Ca2+ channels that is apparently not required for neurotransmitter release; decreases threshold for Na+ channel activation; and slows Na+ channel inactivation. As delta/omega-plectoxin-Pt1a enhances synaptic transmission by prolonging presynaptic release of neurotransmitter, its effects on Na+ and Ca2+ channels may act synergistically to sustain the terminal excitability.