On the Road to Translocase

Protein synthesis is catalyzed by cytosolic ribosomes, yet the proteins themselves are sorted to many distinct subcellular compartments, each a unique membrane or aqueous space bounded by a membrane. To arrive at noncytosolic compartments, proteins must be targeted swiftly and accurately to an intracellular membrane which they can cross or, for integral membrane proteins, into which they can insert. With realization of the lipid bilayer-based mosaic structure of membranes came the twin questions of how insoluble integral membrane proteins could be made by an inherently water-soluble protein-synthesis machinery and how soluble noncytoplasmic proteins could cross the hydrophobic core of a membrane.