The major pathways of protein translocation across membranes.

The initial events in the localization of cell surface proteins include targeting of precursor molecules to the membrane and subsequent translocation across the membrane. The translocation reaction is mediated through a series of molecular interactions involving a number of protein factors. The trimeric SecY/Sec61 core complex, which is conserved throughout evolution, provides a proteinaceous pathway for translocation. The driving force for translocation is provided by the dynamic motion of the SecA ATPase in prokaryotes and by polypeptide elongation through the ribosome-Sec61 junction in eukaryotes.