Membrane protein assembly

Cells are made up of multiple membrane-bound compartments, each containing a unique set of proteins. The sorting and insertion of proteins across and into the many membrane systems of eukaryotes and prokaryotes has been the subject of a vast amount of research. This chapter discusses the assembly of proteins into the cytoplasmic (inner) membrane of prokaryotes, notably Escherichia coll. It is believed that many of the basic aspects of membrane protein assembly in E. coli are relevant to the assembly of proteins into other membrane systems such as the endoplasmic reticular (ER) membrane of eukaryotes, the thylakoid membrane of chloroplasts, and the inner mitochondrial membrane. Knowledge from E. coil can often illuminate the analogous processes in these other systems. The most important characteristics of the substrates for the secretory machinery—the nascent membrane proteins— are clarified with the discovery that positively charged amino acids act as the major topogenic determinants during membrane integration. As a measure of the understanding, one can now predict the topology of an inner membrane protein from its amino acid sequence and expect to be right 9 times out of 10.