Structure CD11a

LFA-1 is an a,(3, noncovalently linked heterodimer, CD1 la/CD18 (Figure 1). Mature human CDlla, a glycoprotein of 180 kDa (126 kDa without glycosylation), consists of a 1063 amino acid extracellular domain including 12 N-linked glycosylation sites, a 29 amino acid transmembrane domain, and a 53 amino acid cytoplasmic domain. The extracellular portion contains seven repeats of roughly 50 amino acids each, five of which are in tandem. The extracellular portion also includes a single -200 amino acid domain similar to domains found in a few non-leukocyte integrins, various receptors found also in invertebrates, some blood proteins, and in collagens and other matrix proteins. This domain is called the A domain because it was the first of five repeated domains, termed simply A through F, identified in von Willebrand factor. In the context of the leukocyte integrins, this same domain is often called the I domain (I for inserted/interactive).

Three of the seven repeated domains include FF-hand-like sequences which are putative divalent cation-binding sites. The I domain includes a different type of divalent cation-binding site called MIDAS (see below). The region containing the I insertion and the repeats has only a single N-linked glycosylation site and a single cysteine, consistent with ligand binding function (see below).

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