Membrane protein topogenesis

University dissertation from Stockholm : Department of Biochemistry and Biophysics, Stockholm University

Abstract: The membranes of cells are highly complex and heterogeneous structures that fulfill multiple vital tasks. They form thin barriers that seal out the environment, thus defining the cell’s boundaries. They mediate the selective exchange of information and substances between the inside and outside of cells, thus making cellular life and survival possible and allowing fast adaptation to changing conditions. Not least importantly, they harbor key components of many essential processes such as the photosynthesis and respiration. Membranes are composed of a largely apolar lipid matrix densely punctuated with a large number of different proteins. These so-called membrane proteins usually span the lipid matrix and protrude out into the space on either side of the membrane.Over millions of years of evolution, cells have developed an incredible machinery to facilitate the insertion of membrane proteins into the membrane. Our understanding of these machines and the insertion processes they mediate has reached a point where we have a very good picture of membrane protein biogenesis in various types of cells. However, more data still needs to be collected to completely comprehend the complex molecular mechanisms and the physical chemistry that underlies the different insertion processes.The work presented in this thesis contributes to that understanding. More precisely, we have studied how weakly hydrophobic transmembrane elements of membrane proteins, which cannot spontaneously enter the largely apolar membrane matrices, are efficiently incorporated. Indeed, such elements are quite common in membrane proteins and our work has lead to the formulation of a novel mechanism for how they can be integrated into biological membranes.We have also added to the understanding of the physical chemistry that underlies the membrane insertion process by systematically introducing non-proteinogenic amino acids into a membrane-spanning segment of a membrane protein and studying its membrane insertion capability.