8.B.21.1.1
Mu-Cyrtautoxin-As1a or ApsIII insecticidal neurotoxin of 37 aas; an atypical knottin.  Blocks votage-gated Na⁺ channels in insects (Skinner et al. 1992).  It acts by pluging the outer vestibule of the channel, and also acts in combination with a weak (30%) voltage-independent block of insect voltage-gated calcium (Cav) channels (low-voltage and high-voltage channels). Tested on DUM neurons, it inhibits sodium currents with an IC50 of 540 nM.  The solution structure is available (PDB# 2M36).