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J. Cell Biol.,
Volume 142, Number 1, July 13, 1998 139-151


* Howard Hughes Medical Institute and Department of Neurobiology, Massachusetts General Hospital and Harvard Medical
School, Boston, Massachusetts 02114; CASK, the rat homolog of a gene (LIN-2) required for vulval differentiation in Caenorhabditis elegans, is expressed in mammalian brain, but its function
in neurons is unknown. CASK is distributed in a punctate somatodendritic pattern in neurons. By immunogold EM, CASK protein is concentrated in synapses,
but is also present at nonsynaptic membranes and in intracellular compartments. This immunolocalization is
consistent with biochemical studies showing the presence of CASK in soluble and synaptosomal membrane
fractions and its enrichment in postsynaptic density
fractions of rat brain. By yeast two-hybrid screening, a
specific interaction was identified between the PDZ domain of CASK and the COOH terminal tail of syndecan-2, a cell surface heparan sulfate proteoglycan
(HSPG). The interaction was confirmed by coimmunoprecipitation from heterologous cells. In brain, syndecan-2 localizes specifically at synaptic junctions where it
shows overlapping distribution with CASK, consistent with an interaction between these proteins in synapses.
Cell surface HSPGs can bind to extracellular matrix
proteins, and are required for the action of various heparin-binding polypeptide growth/differentiation factors.
The synaptic localization of CASK and syndecan suggests a potential role for these proteins in adhesion and
signaling at neuronal synapses.
Department of Cell Biology and Anatomy, University of North Carolina, Chapel Hill,
North Carolina 27599; § Department of Cell Biology and Internal Medicine, Yale School of Medicine, New Haven, Connecticut
06520
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