The effect of calcium levels on synaptic proteins. A study on VAT-1 from Torpedo

In this study we compare major synaptic proteins from Torpedo electric organ to their homologues from mammalian brain. Most of these proteins are members of small gene families. We demonstrate a high degree of evolutionary conservation of most synaptic proteins. However, in the electric organ each gene family is represented only by a single member. We focus on VAT-1, a major protein of the vesicle membrane in Torpedo. VAT-1 is located on the synaptic vesicle membrane and is highly concentrated on the plasma membrane following the application of α-latrotoxin. Taking advantage of the relative simplicity of Torpedo synapses, we performed an in vitro study on the properties of VAT-1 affected by changes in Ca²⁺ levels. VAT-1 is a low affinity Ca²⁺ binding protein whose ability to bind Ca²⁺ resides mainly, but not entirely, on the carboxy-terminal domain of the protein. In the presence of Ca²⁺, the protein is organized in a high molecular mass complex, which is destabilized by depleting Ca²⁺. This effect occurs only by chelating Ca²⁺ ions, but not with other divalent ions. VAT-1 is not complexed to any of the proteins which were implicated in the docking/fusion complex such as VAMP, synaptophysin or syntaxin, regardless of Ca²⁺ levels. Dependence of the stability of protein complexes on Ca²⁺ levels is also demonstrated on Torpedo n-Sec1. The possible physiological implications of such Ca²⁺ dependence are discussed.