CAMSTATINS ARE PEPTIDE ANTAGONISTS OF CALMODULIN BASED UPON A CONSERVED STRUCTURAL MOTIF IN PEP-19, NEUROGRANIN, AND NEUROMODULIN

Unbridled increases in intracellular ionized calcium can result in neu ronal damage and death. Since many of the deleterious effects of calci um are mediated by calmodulin, we have sought to identify neuronal pro teins that inhibit activation of this ubiquitous protein. PEP-19 is a 7.6-kDa neuron-specific protein, which contains a motif similar to the calmodulin binding domains of neuromodulin (GAP 43) and neurogranin ( RC3). Here we show that PEP-19 binds calmodulin in an analogous calciu m-independent manner with an apparent R(d) near 1.2 mu M. Furthermore, using the calmodulin-dependent enzyme neuronal nitric oxide synthase, we demonstrate that native PEP-19 is also an antagonist of enzyme act ivity. Based on the PEP-19 sequence, a series of peptide calmodulin an tagonists termed camstatins were synthesized, These analogs define the minimally active domain of PEP-19 and provide a structure/activity re lationship for calmodulin antagonism. There was a positive correlation between the binding affinities of the camstatins for calmodulin and t heir potencies as neuronal nitric oxide synthase inhibitors. Despite t he similar IQ motif in PEP-19 and neuromodulin or neurogranin, PEP-19 was not a substrate for protein kinase C. The properties of PEP-19 sug gest that it could fulfill a role in calmodulin is present in large ex cess and is not subject to short neuroprotection.