This entry represents domains with an immunoglobulin-like (Ig-like) fold, which consists of a beta-sandwich of seven or more strands in two sheets with a Greek-key topology. Ig-like domains are one of the most common protein modules found in animals, occurring in a variety of different proteins. These domains are often involved in interactions, commonly with other Ig-like domains via their beta-sheets [1, 2, 3, 4]. Domains within this fold-family share the same structure, but can diverge with respect to their sequence. Based on sequence, Ig-like domains can be classified as V-set domains (antibody variable domain-like), C1-set domains (antibody constant domain-like), C2-set domains, and I-set domains (antibody intermediate domain-like). Proteins can contain more than one of these types of Ig-like domains. For example, in the human T-cell receptor antigen CD2, domain 1 (D1) is a V-set domain, while domain 2 (D2) is a C2-set domain, both domains having the same Ig-like fold [5].

Domains with an Ig-like fold can be found in many, diverse proteins in addition to immunoglobulin molecules. For example, Ig-like domains occur in several different types of receptors (such as various T-cell antigen receptors), several cell adhesion molecules, MHC class I and II antigens, as well as the hemolymph protein hemolin, and the muscle proteins titin, telokin and twitchin.

1. The immunoglobulin fold. Structural classification, sequence patterns and common core. J. Mol. Biol. 242, 309-20
2. The immunoglobulin fold family: sequence analysis and 3D structure comparisons. Protein Eng. 12, 563-71
3. Protein--protein recognition: juxtaposition of domain and interface cores in immunoglobulins and other sandwich-like proteins. J. Mol. Biol. 342, 665-79
4. Immunoglobulin superfamily proteins in Caenorhabditis elegans. J. Mol. Biol. 296, 1367-83
5. Crystal structure of the extracellular region of the human cell adhesion molecule CD2 at 2.5 A resolution. Structure 2, 755-66

This entry represents domains with an immunoglobulin-like (Ig-like) fold, which consists of a bet
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