The Immunoglobulin Superfamily

The structures of the various immunoglobulin heavy and light chains described earlier share several features, suggesting that they have a common evolutionary ancestry. In particular, all heavy- and light-chain classes have the immunoglobulin-fold domain structure (see Figure 4-7). The presence of this characteristic structure in all im-munoglobulin heavy and light chains suggests that the genes encoding them arose from a common primordial gene encoding a polypeptide of about 110 amino acids. Gene duplication and later divergence could then have generated the various heavy- and light-chain genes.

Large numbers of membrane proteins have been shown to possess one or more regions homologous to an im-munoglobulin domain. Each of these membrane proteins is classified as a member of the immunoglobulin superfamily. The term superfamily is used to denote proteins whose corresponding genes derived from a common primordial gene encoding the basic domain structure. These genes have evolved independently and do not share genetic linkage or function. The following proteins, in addition to the immunoglobulins themselves, are representative members of the immunoglobulin superfamily (Figure 4-20):

■ Ig-a/Ig-p heterodimer, part of the B-cell receptor

■ Poly-Ig receptor, which contributes the secretory component to secretory IgA and IgM

■ T-cell accessory proteins, including CD2, CD4, CD8, CD28, and the y, 8, and e chains of CD3

■ Class I and class II MHC molecules

■ p2-microglobulin, an invariant protein associated with class I MHC molecules

■ Various cell-adhesion molecules, including VCAM-1, ICAM-1, ICAM-2, and LFA-3

■ Platelet-derived growth factor

Numerous other proteins, some of them discussed in other chapters, also belong to the immunoglobulin superfamily.

X-ray crystallographic analysis has not been accomplished for all members of the immunoglobulin superfamily. Nevertheless, the primary amino acid sequence of these proteins suggests that they all contain the typical immuno-globulin-fold domain. Specifically, all members of the immunoglobulin superfamily contain at least one or more stretches of about 110 amino acids, capable of arrangement

Immunoglobulin (IgM)

Immunoglobulin (IgM)

Superfamily
Adhesion molecules
Immunoglobulin Superfamily

FIGURE 4-20

Some members of the immunoglobulin superfamily, teins. In all cases shown here except for ^-microglobulin, the cara group of structurally related, usually membrane-bound glycopro- boxyl-terminal end of the molecule is anchored in the membrane.

FIGURE 4-20

Some members of the immunoglobulin superfamily, teins. In all cases shown here except for ^-microglobulin, the cara group of structurally related, usually membrane-bound glycopro- boxyl-terminal end of the molecule is anchored in the membrane.

into pleated sheets of antiparallel p strands, usually with an invariant intrachain disulfide bond that closes a loop spanning 50-70 residues.

Most members of the immunoglobulin superfamily cannot bind antigen. Thus, the characteristic Ig-fold structure found in so many membrane proteins must have some function other than antigen binding. One possibility is that the immunoglobulin fold may facilitate interactions between membrane proteins. As described earlier, interactions can occur between the faces of p pleated sheets both of homolo gous immunoglobulin domains (e.g., CH2/CH2 interaction) and of nonhomologous domains (e.g., VH/VL and CH1/CL interactions).

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