Immunoglobulins globular domains

The class 1 MHC structure is divided into two globular regions (Figure 15.18). One region is built up from the two immunoglobulin-like domains a3... 

MHC class II molecules are made up of two membrane-spanning proteins each chain has a size of 30 kDa and is made up of two globular domains. These domains are called a-1, a-2, (3-1 and P-2. Each chain possesses an immunoglobulin-like region next to the cell membrane. MHC class II molecules are highly polymorphic. 

The three-dimensional structure of the PapD periplasmic chaperone that forms transient complexes with pilus subunit proteins has been solved by Holmgren and Branden (1989). PapD consists of two globular domains oriented in the shape of a boomerang (Fig. 2). Each domain is a /3-barrel structure formed by two antiparallel /8-pleated sheets that have a topology similar to an immunoglobulin fold. The relationship between PapD and other immunoglobulin-like proteins is discussed in Section IV,C. 

Figure 4. "Richardson diagrams" [29] of five proteins illustrating four classes of tertiary-fold motifs typically found in globular proteins, (a) AU-a hemoglobin, P subunit, (b) AU- Immunoglobulin variable domain, (c) o/p tnose phosphate isomerase. (d) ot/p alcohol dehydrogenase, domain 2. (e) a+ Stai ylococcal nuclease.

Tertiary structure is the overall folding of the protein molecules, in contrast to the secondary structure, which is the local folding (e.g., a-helix, p-sheets). Tertiary structure makes the protein compact and globular in shape. It may be divided into units called domains. A simple domain contains 100-150 amino acid residues and is about 25 A in diameter. The light chain of IgGl, immunoglobulin, for example, consists of two domains and its heavy chain consists of four domains. In the same molecule, the domains may or may not have the same functions. 

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