Nonsynaptic clusters of postsynaptic proteins has been previously documented; however, their role remains elusive. Here we monitored trafficking of several candidate proteins implicated in synaptogenesis, prior to the majority of synapse formation, and when nonsynaptic clusters of scaffold proteins are most abundant. We find that a protein complex consisting of two distinct subpopulations that differ in their content, mobility, and involvement in synapse formation. One subpopulation is mobile and relies on actin-dependent transport for delivery to nascent and existing synapses. These mobile clusters contain the scaffolding proteins PSD-95 (postsynaptic density-95), GKAP (guanylate kinase associated protein), and Shank but lack the cell adhesion molecule neuroligin-1. The second group consists of stationary nonsynaptic scaffold complexes that contain neuroligin-1 and recruit synaptophysin containing axonal transport vesicles. Sites with stationary complexes are readily transformed to functional presynaptic contacts that recycle the vital dye FM 4-64. These results postulate a novel mechanism whereby preformed scaffold protein complexes serve as predetermined postsynaptic hot spots for establishment of new functional excitatory synapses.
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