Intercellular adhesion molecule-5 (ICAM-5), also known as telencephalin, is a cell surface glycoprotein belonging to the immunoglobulin superfamily. Human ICAM-5 consists of an 832 amino acid (aa) extracellular domain containing 9 immunoglobulin (Ig) domains and 15 N-glycosylation sites, a 28 aa transmembrane domain, and a 64 aa cytoplasmic domain. ICAM-5 shares 38 - 55% aa identity with other ICAMs, being most closely related to ICAM-1 (50% identity) and ICAM-3 (55% identity) (1). Human and mouse ICAM-5 share 85% aa identity.
The tissue distribution of ICAM-5 is unique among ICAMs, being expressed only in telencephalic regions of the central nervous system (2). Like other ICAMs, ICAM-5 binds to the leukocyte integrin LFA-1 (CD11a/CD18) (3). Binding of ICAM-5 to LFA-1 is dependent on the first amino terminal Ig domain of ICAM-5 (4). ICAM-5 also displays homophilic binding, with the amino terminal Ig domain binding to Ig domains 4 - 5. Homophilic binding of ICAM-5 is dependent of ICAM-5 being in a monomeric form. The monomeric form of ICAM-5 is found during dendritogenesis in developing brain, whereas a high molecular weight complex is found in mature neurons (5).
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