Membrane-protein topology

The topology of an integral membrane protein describes the number and approximate locations in the sequence of the transmembrane segments, as well as the overall orientation of the protein in a membrane.Topology is controlled primarily by the hydrophobicity and length of transmembrane helices as well as the distribution of positively charged residues in the loops that connect the helices.In most cases, topology is determined co-translationally during the translocon-mediated insertion of a polypeptide into a membrane.Topologies in which both the N terminus and the C terminus of a protein are in the cytoplasm are predominant in both prokaryotic and eukaryotic cells.Membrane proteins evolve primarily by gene duplication and gene fusion. Many membrane proteins form dimers in which the two homologous chains have the same topology (parallel dimer) or opposite topologies (antiparallel dimer). Gene fusions create internally duplicated structures in which the two halves of a protein are orientated either in a parallel or an antiparallel manner.