Multiple mechanisms of protein insertion into and across membranes.

Protein localization in cells is initiated by the binding of characteristic leader (signal) peptides to specific receptors on the membranes of mitochondria or endoplasmic reticulum or, in bacteria, to the plasma membrane. There are differences in the timing of protein synthesis and translocation into or across the bilayer and in the requirement for a transmembrane electrochemical potential. Comparisons of protein localization in these different membranes suggest underlying common mechanisms.