Membrane proteins which exhibit multiple topological orientations.

Membrane protein folding patterns can be divided into several classes based on their orientation and the topogenic sequences that regulate their insertion into the ER membrane. The orientation of membrane proteins is regulated by charge distribution in the polypeptide chain as well folding characteristics of the N-terminal domain. Protein targeting characteristic are determined by several sequence motifs found in the C- and N-terminal domains as well as by oligomerization. Several proteins, such as Pgp, ductin, CP450s and mEH, have been shown to exhibit more than one topological orientation in the ER which can result in protein targeting to more than one cell compartment as well as the expression of multiple biological functions.