Mechanism of inhibition of calcium uptake into sarcoplasmic reticulum by notexin, a neurotoxic and myotoxic polypeptide.

Notexin belongs to a class of snake venom neurotoxins and myotoxins that have phospholipase A2 activity. Previous studies have shown that these toxins affect target cells differently from phospholipases that are not neurotoxic or myotoxic. Notexin inhibited the Ca2+ uptake into fragmented sarcoplasmic reticulum from rabbit skeletal muscle, but it did not cause an efflux of previously accumulated Ca2+ or inhibit the Ca2+--ATPase activity. It is suggested that notexin specifically binds to and decreases the conductance for Ca2+ of the Ca2+ pump and/or the conductance of a channel for an ion that facilitates Ca2+ transport. The K+ ionophore valinomycin reversed the notexin-induced inhibition of Ca2+ uptake into sarcoplasmic reticulum, suggesting that the molecular target of notexin could be a K+ channel. Two types of reconstitution experiments make it unlikely that notexin acts by degrading a minor lipid that is resistant to hydrolysis by nontoxic phospholipases A2. Notexin-inactivated sarcoplasmic reticulum vesicles were reactivated (with respect to Ca2+ uptake) by simple solubilization with detergent and subsequent reconstitution by detergent removal. Second, notexin was still active on sarcoplasmic reticulum vesicles after greater than 94% of the lipids were replaced by soybean phosphoglycerides during the reconstitution procedure.