The GIT Family of Proteins Forms Multimers and Associates with the Presynaptic Cytomatrix Protein Piccolo*

The cytoskeletal matrix assembled at active zones (CAZ) is implicated in defining neurotransmitter release sites. However, little is known about the molecular mechanisms by which the CAZ is organized. Here we report a novel interaction between Piccolo, a core com-ponent of the CAZ, and GIT proteins, multidomain sig-naling integrators with GTPase-activating protein ac-tivity for ADP-ribosylation factor small GTPases. A small region (150 amino acid residues) in Piccolo, which is not conserved in the closely related CAZ pro-tein Bassoon, mediates a direct interaction with the Spa2 homology domain (SHD) domain of GIT1. Piccolo and GIT1 colocalize at synaptic sites in cultured neu-rons. In brain, Piccolo forms a complex with GIT1 and various GIT-associated proteins, including PIX, foca