Attenuation of cell adhesion in lymphocytes is regulated by CYTIP, a protein which mediates signal complex sequestration

Abstract

An important theme in molecular cell biology is the regulation of protein recruitment to the plasma membrane. Fundamental biological processes such as proliferation, differentiation or leukocyte functions are initiated and controlled through the reversible binding of signaling proteins to phosphorylated membrane components. This is mediated by specialized interaction modules, such as SH2 and PH domains. Cytohesin-1 is an intracellular guanine nucleotide exchange factor, which regulates leukocyte adhesion. The activity of cytohesin-1 is controlled by phosphoinositide-dependent membrane recruitment. An interacting protein was identi®ed, the expression of which is upregulated by cytokines in hematopoietic cells. This molecule, CYTIP, is also recruited to the cell cortex by integrin signaling via its PDZ domain. However, stimulation of Jurkat cells with phorbol ester results in re-localization of CYTIP to the cytoplasm, and membrane detachment of cytohesin-1 strictly requires co-expression of CYTIP. Consequently, stimulated adhesion of Jurkat cells to intracellular adhesion molecule-1 is repressed by CYTIP. These ®ndings outline a novel mechanism of signal chain abrogation through sequestration of a limiting component by speci®c protein±protein interactions.

Cytohesin Binder and Regulator Augments T Cell Receptor-induced Nuclear Factor of Activated T Cells·AP-1 Activation through Regulation of the JNK Pathway

“…Therefore, an alternative possibility is that Cybr may serve as a scaffold protein for the assembly of a multiple complex bringing Vav into proximity with phosphatidylinositol 3-kinase products. Indeed, we and others have shown that Cybr associates with the integrin-binding protein cytohesin-1, another GEF and also a target of phosphatidylinositol 3,4,5-trisphosphate (16,20,21). Cybr has been suggested to regulate cytohesin-1 translocation from the membrane to the cytosol (20,22).…”

“…D, WCLs were prepared and immunoblotted to confirm Cybr expression. scaffold proteins containing protein-protein interaction domains, including an N-terminal PDZ domain and a central CC motif (16,20,21). Its close relative, Tamalin-GRASP, has recently been shown to participate in signal transduction events downstream of the metabotropic glutamate receptors and regulates the expression of these receptors (18,19,34).…”

“…Its close relative, Tamalin-GRASP, has recently been shown to participate in signal transduction events downstream of the metabotropic glutamate receptors and regulates the expression of these receptors (18,19,34). Cybr itself appears to be involved in integrin signaling and the regulation of cell adhesion (20,22). However, whether Cybr was involved in other receptor-mediated events was not known.…”

“…The CC domain of Cybr has been shown to interact with the CC domain in cytohesin-1 (16,20). This interaction results in Cybr regulation of cytohesin-1 GEF activity on its substrate, the ADP-ribosylation factor family GTPases (16).…”

“…Interestingly, in epidermal growth factor-stimulated COS-1 cells overexpressing Cybr and cytohesin, Cybr appears to be recruited from the Golgi compartment to membrane ruffles in a cytohesin-dependent manner (21). Moreover, in Jurkat T cells, Cybr has been shown to drive cytohesin away from membrane-associated integrins to the cytosol and thus attenuate lymphocyte adhesion (20). Using small interfering RNA in dendritic cells, Cybr has been shown to be important for controlling dendritic -T cell interaction (22).…”