Functions of Signaling Machinery
in the Postsynaptic Density

Work on the structure and organization of signaling complexes at the postsynaptic membrane has given us hints about the potential structures and functions of signaling complexes regulated by activation of the NMDA receptor.  However, there are still many intriguing "holes" in our understanding.  We are studying the functions of two prominent molecules that are part of the NMDA receptor complex at most glutamatergic synapses in the CNS.  Both of these molecules were first identified in the PSD fraction, cloned, and sequenced by our lab. 

SYNGAP

The first is SynGAP, a Ras GTPase activating protein that is highly localized to the postsynaptic density, or to small vesicles in the cell body and dendrites that appear to be transport vesicles.  SynGAP is highly brain specific.  In the PSD fraction, it exists as four closely related splice variants, all made from one gene.  Most of the splice variants associate tightly with PSD-95 via its PDZ domains.

SynGAP is rapidly and stoichiometrically phosphorylated by CaM kinase II at two sites.  We are studying the effect of this phosphorylation on SynGAP's location and enzymatic activity.  We have created mutant mice that are missing SynGAP.  These mice die 2 to 4 days after birth.  We are comparing synaptic function in neurons cultured from mutant or wild type animals.

Densin

The second molecule that we are focusing our attention on is densin-180.  We hypothesize that densin may help to localize specific signaling molecules near their targets in the postsynaptic density.  Densin is a transmembrane protein that has a "leucine-rich repeat" interaction domain at its extracellular N-terminus, followed by a long mucin-like region that is highly glycosylated and contains poly-sialic acid.  The transmembrane domain is located in the carboxyl half of the protein, and is followed by a short membrane proximal segment, then an N-terminal PDZ domain.  We found that the cytosolic portion of densin forms a ternary complex with actinin (an actin-associated protein) and CaM kinase II, an important signaling molecule at the postsynaptic site.  We are looking for other proteins that associate with densin, and are creating mutants that we hope will give us insight into densin's function.