The basic structure of immunoglobulin (Ig) molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains: alpha, delta, epsilon, gamma and mu, all consisting of a variable domain (VH) and three (in alpha, delta and gamma) or four (in epsilon and mu) constant domains (CH1 to CH4). Ig molecules are highly modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. The domains in Ig and Ig-like molecules are grouped into four types: V-set, C1-set, C2-set and I-set. Structural studies have shown that these domains share a common core Greek-key beta-sandwich structure, with the types differing in the number of strands in the beta-sheets as well as in their sequence patterns.
Immunoglobulin-like domains that are related in both sequence and structure can be found in several diverse protein families. Ig-like domains are involved in a variety of functions, including cell-cell recognition, cell-surface receptors, muscle structure and the immune system.
Studies indicate that the interactions essential for defining the structure of these beta sandwich proteins are also important in nucleation of folding, and that proteins containing this fold may share similar folding pathways even though the proteins may have low sequence homology. The fold consists of a beta-sandwich formed of 7 strands in 2 sheets with a Greek-key topology. Some members of the fold have additional strands. The Pfam alignments do not include the first and last strand of the immunoglobulin-like domain.