Long C terminal splice variant CaV2.2 identified in presynaptic membrane by mass spectrometric analysis

Ca(V)2.2 voltage-gated calcium channels play a key role in the gating of transmitter release at presynaptic terminals. Recently we used mass spectrometry (MS) to analyze the protein complex associated with Ca(V)2.2 in purified presynaptic terminal membranes. A number of known and new Ca(V)2.2-associated proteins were identified, but not the channel itself. Here we set out to explore this anomaly. As previously, we used antibody Ab571 to capture the channel from purified synaptosome membrane lysate. We prepared a brain membrane lysate enriched for presynaptic active zones using standard methods to fractionate purified synaptosomes. These were osmotically lysed to generate a fraction enriched in presynaptic surface membranes. The lysate was solubilized in modified RIPA buffer and was passed over anti-Ca(V)2.2 antibody covalently bonded to immunoprecipitation beads. Captured complexes on the beads were then stripped of weakly-bound proteins by exposure to high salt to enrich the channel fraction. Proteins remaining bound to the sample were recovered in high concentration urea and the sample was subjected to standard enzyme digestion and MS analysis. We identified 12 distinct Ca(V)2.2 peptides, but no other ion channel peptides, in the lysate-exposed bead sample but no other ion channel peptides were recovered. Interestingly one of the channel peptides was derived from the alternatively spliced, long-C terminal region. Hence, confidence in identification of Ca(V)2.2 was beyond reasonable doubt. The identification of the long-splice Ca(V)2.2 provides compelling evidence that this variant is targeted to the presynaptic terminal, as we and others have suggested.