Are proteins translocated through pores? An historical overview.

Most proteinaceous pores are characterized as ionic channels. However, some are also involved in protein translocation through phospholipidic membranes. This concept has evolved slowly in cell biology and in biophysics, requiring the development of adapted electrical and biochemical methods. Protein translocation in mitochondria biogenesis, secretion by endoplasmic reticulum or bacteria, and bacterial toxins internalization are the main fields where proteinconducting pores have been described. The concept is now well established and progress at the molecular and atomic levels have shown how different this paradigm is from ionic channels involved in neurobiology. Protein-conducting pores are often parts of large complexes and electrical analysis gives on-line information at the single-molecule level. They have a large conductance that, in certain membranes, should be highly regulated to prevent ionic leaking through the membrane. Finally, they are involved not only in protein translocation, but also in membrane protein insertion (α-helix and β-barrel types).