Class I and class II MHC molecules

Corresponding to the general organization of
the individual class I and class II genes, the class
I and class II molecules are basically similar.
Both consist of two different polypeptide
chains. In class I molecules, an MHC-coded !
chain is associated with a non-MHC-coded "
chain ("2-microglobulin). The extracellular portion
of the ! chain consists of three domains,
!3, !2, and !1, each with about 90 amino acids.
!1 and !2 form the highly polymorphic peptide-
binding region; !3 and "2-microglobulin
structurally correspond to an immunoglobulinlike
region. Elucidation of the crystalline structure
of the class I MHC molecules showed that
!1 and !2 interact to form a type of platform of
eight-stranded "-folded proteins. The cleft
formed between !1 and !2 (25 Å!10 Å!11 Å)
can bind a protein fragment consisting of
10–20 amino acids. Class II MHC molecules
consist of two polypeptide chains, ! and ", each
with two domains, i.e., !1, !2 and "1, "2, each
with about 90 amino acids and a transmembrane
region of about 25 amino acids. As with
the class I molecules, the peptide-binding regions
(!1 and "1) are highly polymorphic. Unlike
the "1 domain, !1 does not contain a disulfide
bridge.