Class Ii Mhc Proteins

MHC class II proteins are found constitutively on B cells and dendritic cells and can be upregulated on macrophages and Langerhans cells. Each molecule is composed of two noncovalently bound a and B chains (Fig. 2). Although both chains have the same structure, the a chain is slightly heavier (32-34 kDa) because of extensive glycosylation. Like the class I protein, the class II protein consists of a peptide-binding region, an immunoglobulin-like domain, and transmembrane and cytoplasmic regions.

In contrast to the class I marker, the peptide-binding portion is composed of the al and fil region of each chain. These regions interact to form an eight-stranded B-pleated sheet with two supporting a-helices. The ends of the peptide region are open allowing bound peptides to extend into the external milieu.

Figure 2. Schematic representation of the MHC class II protein. N and C refer to amino and carboxy termini of the polypeptide chain, S-S to intrachain polypeptide bonds, and P to phosphorylation sites. Reprinted from Cellular and Molecular Immunology, Abbas et al., 1994, with permission of W. B. Saunders Co.

Although the optimal binding length is 10-14 amino acids, peptides as long as 30 amino acids have been recovered from purified class II molecules.

Distal to the peptide-binding regions are the a2 and B2 immunoglobulin-like regions. Both domains have a loop structure linked by disulfide bonds. Because of homologies in amino acid sequence, a2 and B2 domains belong to the Ig superfamily. These domains are important in establishing noncovalent interactions between chains. Like other transmembrane glycoproteins, there are short transmembrane regions of 25 hydrophobic amino acids that span the cytoplasmic membrane. Attached to the cluster of basic amino acids at the terminus of the transmembrane region are short hydrophilic cytoplasmic tails of variable length (Schafer etal., 1995).

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