Role of adhesion molecule cytoplasmic domains in mediating interactions with the cytoskeleton.

The past ten years have seen significant progress in cell biology research aimed at understanding how cytoskeletal filaments interact with the plasma membrane. Considerable evidence suggests that both actin microfilaments and intermediate filaments attach to the membrane via the cytoplasmic domains of various membrane proteins including adhesion molecules. Interactions between the cytoskeleton and adhesion molecules appear to be essential for a variety of cellular functions, including cell-cell and cell-extracellular matrix (ECM) interactions, cell motility, receptor-ligand interactions, and receptor internalization. Recently, many of the detailed molecular mechanisms which mediate the associations between actin filaments and adhesion molecules have been identified. Among adhesion molecules that support the attachment of cytoskeletal filaments to their cytoplasmic domains are members of the integrin and cadherin families, the intracellular adhesion molecule-1 (ICAM-1, an immunoglobulin family member), and the glycoprotein Ib/IX complex in platelets. A general conclusion emerging from these studies is that physical associations between cytoskeletal filaments and transmembrane glycoproteins do not occur directly between the filaments and the cytoplasmic tails of adhesion molecules. Instead, these interactions appear to be indirect and involve a complex ensemble of intermediary linker proteins. The severe effects of cytoplasmic domain deletion and mutagenesis on adhesion-dependent functions support the view that receptor cytoplasmic domains play a vital role in regulating receptor function and in mediating communication across the membrane. Transfection studies with mutant and chimeric adhesion molecules, along with protein-binding studies, are clarifying the mechanisms which physically link the cytoskeleton to transmembrane proteins, regulate cytoskeletal organization, mediate signaling across the cell membrane, and regulate the ligand specificity and binding affinity of surface receptors.