M‐calpain‐mediated cleavage of GAP‐43 near Ser41 is negatively regulated by protein kinase C, calmodulin and calpain‐inhibiting fragment GAP‐43‐3

Neuronal protein GAP‐43 performs multiple functions in axon guidance, synaptic plasticity and regulation of neuronal death and survival. However, the molecular mechanisms of its action in these processes are poorly understood. We have shown that in axon terminals GAP‐43 is a substrate for calcium‐activated cysteine protease m‐calpain, which participates in repulsion of axonal growth cones and induction of neuronal death. In pre‐synaptic terminals in vivo, in synaptosomes, and in vitro, m‐calpain cleaved GAP‐43 in a small region near Ser41, on either side of this residue. In contrast, µ‐calpain cleaved GAP‐43 in vitro at several other sites, besides Ser41. Phosphorylation of Ser41 by protein kinase C or GAP‐43 binding to calmodulin strongly suppressed GAP‐43 proteolysis by m‐calpain. A GAP‐43 fragment, lacking about forty N‐terminal residues (named GAP‐43‐3), was produced by m‐calpain‐mediated cleavage of GAP‐43 and inhibited m‐calpain, but not µ‐calpain. This fragment prevented complete cleavage of intact GAP‐43 by m‐calpain as a negative feedback. GAP‐43‐3 also blocked m‐calpain activity against casein, a model calpain substrate. This implies that GAP‐43‐3, which is present in axon terminals in high amount, can play important role in regulation of m‐calpain activity in neurons. We suggest that GAP‐43‐3 and another (N‐terminal) GAP‐43 fragment produced by m‐calpain participate in modulation of neuronal response to repulsive and apoptotic signals.