C H A P T E R 3 5 , F I G U R E 1 3

Major histocompatibility complex (MHC) proteins class

I.

The MHC class

I

proteins

of antigen-presenting cells comprise two polypeptide chains,

a

and p. The a-polypeptide (-360 residues) (helices red,

(3 sheets dark yellow, and loops gray) has two extracellular domains shown here (cel and

a.2)

and a transmembrane and

intracellular domain (a3, not shown). The a3 domain is structurally similar to the immunoglobulins. The non- MHC

gene-related protein, |32-microglobulin (99 residues), is shown in green on the right side of each structure. The other

subunits of the complex that are involved in T-cell receptor recognition are not shown. The figures are derived from the

coordinates published in the Protein Data Bank files 1AGD and 1AGB. (A) HIV-1 GAG peptide GGKKKYKL present-

ed on the surface of the MHC class II protein. The peptide is shown with the residues “space-filled”, the Lys residue, K

is colored cyan and marked by the arrow. (B) HIV-1 GAG peptide GGRKKYKL presented on the surface of the MHC

class II protein. The peptide is shown with the residues space-filled, the Arg residue, R is colored cyan and marked by

the arrow. Note that the position of the a helix (part of the cul domain) immediately adjacent to the Arg or Lys residues

has undergone a change in position (conformation change) and has a small helical segment (lower left) as a result of

this single-amino-acid substitution. Such sensitivity to small changes in antigen structure is critical to specificity.

C H A P T E R 3 5 , F I G U R E 1 4

Complex of MHC class

I

proteins,

C H A P T E R 3 5 , F IG U R E 15

Major histocompatibility complex (MHC)

HTLV-1 peptide (antigen) and TcR. The MHC class I proteins bind and

proteins class II. The MHC class II proteins bind and present and present

present peptides that are synthesized inside the infected cell to T-cell

peptides that are synthesized outside the infected cell, i.e., peptides that are

receptors (TcRs) specific for the bound peptides. Interaction and stimula-

derived from proteins of the infecting organisms, bound to their specific T-

tion of T-cell proliferation occurs with CD8 positive, cytoxic Tc cells.

cell receptors (TcRs). Interaction and stimulation of T-cell proliferation

The figure is derived from the coordinates published in the Protein Data

occurs with CD4+ positive T cells (Figure 35-16). The figure is derived from

Bank file 1BD2.

the coordinates published in the Protein Data Bank file 1A6A.