The presynaptic calcium channel is part of a transmembrane complex linkinga synaptic laminin (alpha 4 beta 2 gamma 1) with non-erythroid spectrin

Nerve regeneration studies at the neuromuscular junction (NMJ) suggest that synaptic basal lamina components tell the returning axon where to locate n eurotransmitter release machinery, including synaptic vesicle clusters and active zones. Good candidates for these components are the synaptic laminin s (LNs) containing alpha 4, alpha 5, or beta 2 chains. Results from a beta 2 laminin knockout mouse have suggested a linkage of this extracellular lam inin to cytosolic synaptic vesicle clusters. Here we report such a transmem brane link at the electric organ synapse, which is homologous to the NMJ. W e immunopurified electric organ synaptosomes and found on their surface two laminins of 740 and 900 kDa. The 740 kDa laminin has a composition of alph a 4 beta 2 gamma 1 (laminin-9). Immunostaining reveals that as in the NMJ, alpha 4 and beta 2 chains are concentrated at the electric organ synapse. U sing detergent-solubilized synaptosomes, we immunoprecipitated a complex co ntaining alpha 4 beta 2 gamma 1 laminin, the voltage-gated calcium channel, and the cytoskeletal protein spectrin. Other presynaptic proteins such as 900 kDa laminin are not found in this complex. We hypothesize that alpha 4 beta 2 gamma 1 laminin in the synaptic basal lamina attaches to calcium cha nnel, which in turn is attached to cytosolic spectrin. Spectrin could then organize synaptic vesicle clusters by binding vesicle-associated proteins.