Protein Translocation across Membranes

Abstract

Cells are divided into compartments that serve to separate and coordinate metabolic processes. In eukaryotic cells, almost all enzymes, along with proteins involved in their maintenance, are encoded by genes in the nucleus. Proteins are translated in the cytoplasm, and must be targeted to the correct compartment within the cell in order to faithfully carry out their function. To achieve this, proteins are synthesized with signal sequences that guide them to receptor proteins that in turn transfer them to large transmembrane protein channels of organelles. The molecular machinery for protein translocation has evolved to capture and recognize proteins with the correct targeting signals encoded in their amino acid sequence. These diverse classes of translocases pass substrate proteins across, or into, membranes cotranslationally or posttranslationally. Protein substrates are moved by active pushing, pulling, electrophoretic and Brownian ratchet mechanisms. Most translocases operate on unfolded polypeptides, whereas some can transport partially or fully folded proteins. Key Concepts: Cells contain sophisticated molecular machines to recognize and translocate proteins across membranes and into their correct organelles. Proteins are targeted to subcellular locations via targeting signals encoded in their amino acid sequence. These usually exist as linear sequences in the N ‐ or C ‐terminal region of the protein; however, some targeting signals occur within internal segments of the substrate polypeptide. Not all targeting signals are well defined. Receptors associated with the translocase and in the cytosol specifically recognize targeting signals. Translocases contain pores through which the substrate proteins can pass. Most translocase pores are only large enough to accommodate unfolded proteins, although some appear to be able to accommodate fully folded polypeptides. Molecular chaperones maintain substrates in an unfolded state before translocation and assist their correct folding after translocation. The force required to translocate proteins across membranes can come from several sources; the electrophoretic potential or pH gradient across the membrane, an ATP‐ or GTP‐driven pushing or pulling mechanism and an ATP‐driven Brownian ratchet mechanism.