Peripheral plasma membrane protein CASK is a protein that in humans is encoded by the CASK gene.

This gene encodes a calcium/calmodulin-dependent serine protein kinase. The encoded protein is a MAGUK (membrane-associated guanylate kinase) protein family member. These proteins are scaffold proteins and the encoded protein is located at synapses in the brain. Mutations in this gene are associated with FG syndrome 4, mental retardation and microcephaly with pontine and cerebellar hypoplasia, and a form of X-linked mental retardation. Multiple transcript variants encoding different isoforms have been found for this gene.[provided by RefSeq, Mar 2010].
Function: Multidomain scaffolding protein with a role in synaptic transmembrane protein anchoring and ion channel trafficking. Contributes to neural development and regulation of gene expression via interaction with the transcription factor TRB1. Binds to cell-surface proteins, including amyloid precursor protein, neurexins and syndecans. May mediate a link between the extracellular matrix and the actin cytoskeleton via its interaction with syndecan and with the actin/spectrin-binding protein 4.1.