Insertion and Topology of Inner Membrane Proteins in Escherichia coli

Abstract: Integral membrane proteins are found in all cellular membranes and fulfil many of the functions that are central to life. They maintain the membrane structure, but are also involved in the regulation of different cellular processes. All membrane proteins destined for the plasma membrane or that are to be secreted are synthesised in the cytoplasm, and to perform their function they have to be transported to the membrane and correctly inserted into the membrane.Protein trafficking within the cell is complex and has to be precisely controlled. In the Gram-negative bacterial cell, there are five different compartments: the cytoplasm, the inner- and outer membranes, the periplasm and the cell-exterior, and in the eukaryotic cell with its different organelles, there are even more compartments for a protein to end up in. Newly synthesised polypeptide chains are made with an address-tag, the signal peptide, that is recognised by factors in the cytoplasm and thus allowing targeting of the polypeptide chain to the correct membrane.In this thesis, research has been focused on integral membrane proteins destined for the bacterial inner membrane. Questions concerning targeting of membrane proteins, their insertion into the membrane, and finally their orientation within the membrane are addressed. I have analysed the degree of sec-dependence and the assembly properties for the E.coli inner membrane protein MalF. Topological studies have been performed on YidC, a novel E.coli protein homologues to the mitochondrial Oxa1p. An additional topological study has been performed, but from an evolutionary perspective, on two E.coli proteins, ORF193 and YdgQ. Although, they belong to the same family and are strongly related, they have opposite topologies which could suggest that evolution can impart different membrane topologies on strongly related proteins by reshuffling of positively charged residues. Detailed studies of the characterisation of transmembrane segments and turn formation in membrane proteins are also reported.