Peptides transmembrane polypeptide

There are a number of MHC class I and class II polymorphic molecules, all with substantially the same basic structure. The MHC class I molecule is a dimer consisting of a glycoslylated transmembrane peptide of molecular weight 45 kDa covalently linked to a 12 kDa peptide it is found on the surface of most nucleated cells within the body. It is believed that the polypeptide backbones fold in such as way as to form a platform of -pleated sheet structures to support a peptide binding cleft in which the antigen fragments are held and presented to the T cells. 

Proteins are targeted to various locations after synthesis by signal sequences. Thus, proteins destined for the ER, the mitochondria and chloroplasts have particular kinds of signal sequences at the N-terminus. ER-targeted proteins enter the ER directly off rough ER ribosomes via a signal recognition particle (SRP) complex that is linked to an SRP receptor and a ribosome receptor-transmembrane peptide translocation complex associated with the ER membrane. Within, the ER polypeptides are processed and folded and S—S links are formed. 

Signal sequences represent a larger class of polypeptide sequences which share the characteristic of performing their functions by virtue of gross structural features and physical behavior in distinct environments. Other examples most likely include transmembrane sequences, viral fusion sequences, membrane entry sequences in toxins, and signal peptides of mitochondria and chloroplasts. These sequences are unlike seg-... 

The overall structure of a class IT VIHC molecule is remarkably similar to that of a class I molecule. Class II molecules consist of a 33-kd cx chain and a noncovalently bound 30-kd (3 chain (Figure 33.36). Each contains two extracellular domains, a transmembrane segment, and a short cytoplasmic tail. The peptide-binding site is formed by the and 3i domains, each of which contributes a long helix and part of a (3 sheet. Thus, the same structural elements are present in class I and class II VIHC molecules, but they are combined into polypeptide chains in different ways. The peptide-binding site of a class IT molecule is open at both ends, and so this groove can... 

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