Class I Molecules Have a Glycoprotein Heavy Chain and a Small Protein Light Chain

Class I MHC molecules contain a 45-kilodalton (kDa) a chain associated noncovalently with a 12-kDa ^-microglobulin molecule (see Figure 7-5). The a chain is a transmembrane glycoprotein encoded by polymorphic genes within the A, B, and C regions of the human HLA complex and within the K and D/L regions of the mouse H-2 complex (see Figure 7-1). ^-Microglobulin is a protein encoded by a highly conserved gene located on a different chromosome. Association of the a chain with ^-microglobulin is required for expression of class I molecules on cell membranes. The a chain is anchored in the plasma membrane by its hydrophobic transmembrane segment and hydrophilic cytoplasmic tail.

Structural analyses have revealed that the a chain of class I MHC molecules is organized into three external domains (a1, a2, and a3), each containing approximately 90 amino acids; a transmembrane domain of about 25 hydrophobic amino acids followed by a short stretch of charged (hy-drophilic) amino acids; and a cytoplasmic anchor segment of 30 amino acids. The ^-microglobulin is similar in size and organization to the a3 domain; it does not contain a transmembrane region and is noncovalently bound to the class I glycoprotein. Sequence data reveal homology between the a3

Class I molecule

Class II molecule

Membrane - distal domains

Membrane-proximal «3 domains (Ig-fold structure)

Transmembrane segment Cytoplasmic tail

FIGURE 7-5

Class I molecule

Membrane-proximal «3 domains (Ig-fold structure)

Transmembrane segment Cytoplasmic tail

Schematic Diagram Class Molecules
Peptide-binding cleft

ß2-microglobulin a2

Class II molecule

ß2-microglobulin a2

Mhc Class Transmembrane
ß1

Schematic diagrams of a class I and a class II MHC molecule showing the external domains, transmembrane segment, and cytoplasmic tail. The peptide-binding cleft is formed by the membrane-distal domains in both class I and class II molecules. The membrane-proximal domains possess the basic immunoglobulin-fold structure; thus, class I and class II MHC molecules are classified as members of the immunoglobulin superfamily.

Peptide-binding cleft

a1 domain

ß2-microglobulin

Peptide-binding cleft a1 domain

ß2-microglobulin

Mhc Molecule Crystallography

a2 domain a 3 domain

Representations of the three-dimensional structure of the external domains of a human class I MHC molecule based on x-ray crystallographic analysis. (a) Side view in which the p strands are depicted as thick arrows and the a helices as spiral ribbons. Disulfide bonds are shown as two interconnected spheres. The a1 and a2 domains interact to form the peptide-binding cleft. Note the im-

a2 domain a 3 domain a1 domain

a1 domain

Helix Domain

a helix

ß sheets a helix

FIGURE 7-6

Representations of the three-dimensional structure of the external domains of a human class I MHC molecule based on x-ray crystallographic analysis. (a) Side view in which the p strands are depicted as thick arrows and the a helices as spiral ribbons. Disulfide bonds are shown as two interconnected spheres. The a1 and a2 domains interact to form the peptide-binding cleft. Note the im-

munoglobulin-fold structure of the a3 domain and ß2-microglobulin. (b) The a1 and a2 domains as viewed from the top, showing the peptide-binding cleft consisting of a base of antiparallel ß strands and sides of a helices. This cleft in class I molecules can accommodate peptides containing 8-10 residues.

domain, p2-microglobulin, and the constant-region domains in immunoglobulins. The enzyme papain cleaves the a chain just 13 residues proximal to its transmembrane domain, releasing the extracellular portion of the molecule, consisting of a1, a2, a3, and p2-microglobulin. Purification and crystallization of the extracellular portion revealed two pairs of interacting domains: a membrane-distal pair made up of the a 1 and a2 domains and a membrane-proximal pair composed of the a3 domain and p2-microglobulin (Figure 7-6a).

The a1 and a2 domains interact to form a platform of eight antiparallel p strands spanned by two long a-helical regions. The structure forms a deep groove, or cleft, approximately 25 A X 10 A X 11 A, with the long a helices as sides and the p strands of the p sheet as the bottom (Figure 7-6b). This peptide-binding cleft is located on the top surface of the class I MHC molecule, and it is large enough to bind a peptide of 8-10 amino acids. The great surprise in the x-ray crystallo-graphic analysis of class I molecules was the finding of small peptides in the cleft that had cocrystallized with the protein. These peptides are, in fact, processed antigen and self-pep-tides bound to the a1 and a2 domains in this deep groove.

The a3 domain and p2-microglobulin are organized into two p pleated sheets each formed by antiparallel p strands of amino acids. As described in Chapter 4, this structure, known as the immunoglobulin fold, is characteristic of im-munoglobulin domains. Because of this structural similarity, which is not surprising given the considerable sequence similarity with the immunoglobulin constant regions, class I MHC molecules and p2-microglobulin are classified as members of the immunoglobulin superfamily (see Figure 4-20). The a3 domain appears to be highly conserved among class I MHC molecules and contains a sequence that interacts with the CD8 membrane molecule present on TC cells.

p2-Microglobulin interacts extensively with the a3 domain and also interacts with amino acids of the a1 and a2 domains. The interaction of p2-microglobulin and a peptide with a class I a chain is essential for the class I molecule to reach its fully folded conformation. As described in detail in Chapter 8, assembly of class I molecules is believed to occur by the initial interaction of p2-microglobulin with the folding class I a chain. This metastable "empty" dimer is then stabilized by the binding of an appropriate peptide to form the native trimeric class I structure consisting of the class I a chain, p2-microglobulin, and a peptide. This complete molecular complex is ultimately transported to the cell surface.

In the absence of p2-microglobulin, the class I MHC a chain is not expressed on the cell membrane. This is illustrated by Daudi tumor cells, which are unable to synthesize p2-microglobulin. These tumor cells produce class I MHC a chains, but do not express them on the membrane. However, if Daudi cells are transfected with a functional gene encoding p2-microglobulin, class I molecules appear on the membrane.

Light Chain Mhc
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Responses

  • alfrida goodchild
    Do mhc class molecules have constant gegions?
    8 years ago

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