Untitled - D Ank Unlimited

class I region 175 class II MHC molecules
α1
β2m
C
Three-dimensional structure of the external domains of a human class I
HLA molecule based on x-ray crystallographic analysis. The β strands
are depicted as thick arrows and the α helices as spiral ribbons. Disulfide
bonds are shown as two interconnected spheres. The α 1 and α 2 domains
interact to form the peptide-binding cleft. Note the immunoglobulin fold
structure of the α 3 and β 2 microglobulin.
chain of MHC class I in the membrane. The carboxyl terminal
region emerges as a 30-amino acid stretch in the cytoplasm.
Class I histocompatibility antigens are products of the MHC
locus. HLA-A, -B, and -C genes located in the MHC region
on the short arm of chromosome 6 in humans encode these
molecules. K, D, and L genes located on chromosome 17 in
the H-2 complex in mice encode murine MHC class I antigens.
The Tla complex situated near H-2 encodes additional class
I molecules in mice. In T-cell-mediated cytotoxicity, CD8 + T
lymphocytes kill antigen-bearing target cells. The cytotoxic T
lymphocytes play a significant role in resistance to viral infection.
MHC class I molecules present viral antigens to CD8 + T
lymphocytes as a viral peptide class I molecular complex that
is transported to the infected cell surface. Cytotoxic CD8 + T
cells recognize this and lyse the target before the virus can
replicate, thereby stopping the infection.
class I region
That part of the major histocompatibility complex (MHC)
that contains the MHC class I heavy chain genes.
0
N
DPA2
DPA1
DPB2 DPB1
N
C
α2
DNA
α3
DMB
DMA
500
class II antigens
Major histocompatibility complex (MHC) antigens with
limited distribution on such cells as B lymphocytes and
macrophages. In humans, these antigens are encoded by
genes at the DR, DP, and DQ loci.
class-II-associated invariant chain peptide (CLIP)
A peptide of variable length cleaved from the class II variant
chain by proteases. It resides in the major histocompatibility
complex (MHC) class II peptide-binding cleft and
remains associated with the MHC class II molecules in an
unstable form until removed by the HLA-DM protein.
class II MHC molecules
Glycoprotein histocompatibility antigens that play a critical
role in immune system cellular interactions. Each major
histocompatibility complex (MHC) class II molecule is
comprised of a 32- to 34-kDa α chain and a 29- to 32-kDa
β chain, each of which possesses N-linked oligosaccharide
groups, amino termini that are extracellular, and carboxyl
termini that are intracellular. Approximately 70%
of both α and β chains are extracellular. Separate MHC
genes encode the class II molecule α and β chains that are
polymorphic. Class II molecules resemble class I molecules
structurally as revealed by class II molecule nucleotide and
amino acid sequences. MHC class II molecules consist of
a peptide-binding region, a transmembrane segment, and
an intracytoplasmic portion. The extracellular portions of
α and β chains consist of α-1 and α-2 and β-1 and β-2 segments,
respectively. The α-1 and α-2 segments constitute
the peptide-binding region and consist of approximately
90 amino acid residues each. The immunoglobulin-like
region is comprised of α-2 and β-2 segments folded into
immunoglobulin domains in the class II molecule. The
transmembrane region consists of approximately 25 hydrophobic
amino acid residues. The transmembrane portion
ends with a group of basic amino acid residues immediately
followed by hydrophilic tails that extend into the cytoplasm
and constitute the carboxyl terminal ends of the chains.
The α chain is more heavily glycosylated than the β chain.
Of the five exons in the α genes, one encodes the signal
sequence and two code for the extracellular domains. The
transmembrane domain and a portion of the 3′ untranslated
segment are encoded by a fourth exon. The remaining part
of the 3′ untranslated region is coded for by a fifth exon.
Six exons are present in the β genes. They resemble α
gene exons 1 through 3. The transmembrane domain and a
portion of the cytoplasmic domain are encoded by a fourth
exon, the cytoplasmic domain is coded for by the fifth exon,
and the sixth exon encodes the 3′ region that is untranslated.
B lymphocytes, macrophages, or other accessory
cells express MHC class II antigens. Interferon-γ or other
agents may induce an aberrant expression of class II antigen
LMP
LMP
TAP DQA2 DQB1
TAP DOB DQB2 DQB3 DQA1
Class II MHC.
Proteasome
genes
DRB
DRA
1000
C