TYROSINE PHOSPHORYLATION MODULATES THE ACTIVITY OF CLOSTRIDIAL NEUROTOXINS

Clostridial neurotoxins' metalloprotease domain selectively cleaves pr oteins implicated in the process of synaptic vesicle fusion with the p lasma membrane and, accordingly, blocks neurotransmitter release into the synaptic cleft, Here we investigate the potential modulation of th ese neurotoxins by intracellular cascades triggered by environmental s ignals, which in turn may alter its activity on target substrates. We report that the nonreceptor tyrosine kinase Src phosphorylates botulin um neurotoxins A, B, and E and tetanus neurotoxin. Protein tyrosine ph osphorylation of serotypes A and E dramatically increases both their c atalytic activity and thermal stability, while dephosphorylation rever ses the effect. This suggests that the biologically significant form o f the neurotoxins inside neurons is phosphorylated. Indeed, in PC12 ce lls in which tyrosine kinases such as Src and PYK2 are highly abundant , stimulation by membrane depolarization in presence of extracellular calcium induces rapid and selective tyrosine phosphorylation of intern alized light chain, the metalloprotease domain, of botulinum toxin A. These findings provide a conceptual framework to connect intracellular signaling pathways involving tyrosine kinases, G-proteins, phosphoino sitides, and calcium with the action of botulinum neurotoxins in abrog ating vesicle fusion and neurosecretion.