Membrane proteins come in numerous types with a few different suggested classifications. One of the most commonly used to date is the classification method suggested by JS Singer: Type I proteins have a single TM stretch of
hydrophobic residues, with the portion of the polypeptide on the NH2-terminal side of the TM domain exposed on the exterior side of the
membrane and the COOH-terminal portion exposed on the cytoplasmic side. The proteins are subdivided into types Ia (cleavable signal sequences) and Ib (without cleavable signal sequence). Most eukaryotic mebrane proteins with single spanning regions are of Type Ia. Type II membrane proteins are similar to the type I class in that they span the membrane only once, but they have their amino terminus on the cytoplasmic side of the cell and the carboxy terminus on the exterior. Type III membrane proteins have multiple transmembrane domains in a single polypeptide chain (See also figure). They are also sub divided into a and b: Type IIIa molecules have cleavable signal sequences while type IIIb have their amino termini exposed on the exterior surface of the membrane, but do not have a cleavable signal sequences. Type IIIa proteins include the M and L peptides of the photoreaction center. Type IIIb proteins include e.g. cytochrome P450, and leader peptidase of E. coli.
Type IV proteins have multiple homologous domains which make up an assembly that spans the membrane multiple times. The domains may reside on a single polypeptide chain or be on more than one indivdual chain.