Class II MHC molecules contain two different polypeptide chains, a 33-kDa a chain and a 28-kDa p chain, which associate by noncovalent interactions (see Figure 7-5b). Like class I a chains, class II MHC molecules are membrane-bound glycoproteins that contain external domains, a transmembrane segment, and a cytoplasmic anchor segment. Each chain in a class II molecule contains two external domains: a1 and a2 domains in one chain and pi and p2 domains in the other. The membrane-proximal a2 and p2 domains, like the membrane-proximal a3/p2-microglobulin domains of class I MHC molecules, bear sequence similarity to the im-munoglobulin-fold structure; for this reason, class II MHC molecules also are classified in the immunoglobulin super-family. The membrane-distal portion of a class II molecule is composed of the ai and pi domains and forms the antigen-binding cleft for processed antigen.
X-ray crystallographic analysis reveals the similarity of class II and class I molecules, strikingly apparent when the molecules are surperimposed (Figure 7-7). The peptide-binding cleft of HLA-DR1, like that in class I molecules, is composed of a floor of eight antiparallel p strands and sides of antiparallel a helices. However, the class II molecule lacks the conserved residues that bind to the terminal residues of short peptides and forms instead an open pocket; class I presents more of a socket, class II an open-ended groove. These functional consequences of these differences in fine structure will be explored below.
An unexpected difference between crystallized class I and class II molecules was observed for human DR1 in that the rf7 Ja.
Was this article helpful?
All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.