Lipid involvement in protein translocation

In membrane biogenesis and protein targeting, polypeptide chains very often insert into and move across biological membranes. Correct membrane assembly and functioning critically depends on the spatial and temporal interactions between the two main membrane components, i.e. proteins and lipids. It can therefore a priori be reasoned that protein-lipid interactions must be involved in membrane insertion and translocation of newly synthesized proteins. Such an involvement will be of a general nature such as to correctly assemble a putative proteinaceous insertion/translocation device in a membrane and to provide and maintain the essential barrier function of the membrane during the translocation process. In addition, membrane lipids could play more direct roles to provide alone or in combination with proteins an insertion and translocation pathway. The specific, dynamical, and complex lipid composition of biomembranes together with the unique structural and motional properties of individual membrane lipid classes offer fascinating possibilities for protein-lipid interactions to be involved in binding, insertion, translocation and release of proteins in transit across membranes. The approach presented in the chapter to analyze these possibilities is to study selected protein transport pathways in the main cellular protein trafficking routes via a combination of biochemical and biophysical techniques using both model and biological membranes. This chapter summarizes the current insights into the involvement of lipids in protein translocation, in protein secretion of prokaryotes, and in protein import in mitochondria and chloroplasts.