Models for transmembrane translocation of proteins.

Various models for the transmembrane translocation of secreted and membrane-spanning proteins are reviewed in the light of available experimental data. Particular emphasis is placed on the crucial distinction between models where the protein passes through a hydrophilic 'tunnel' through the membrane, and models where translocation takes place through the non-polar intramembranous environment. It is concluded that rather simple reasoning in terms of a protein's behaviour in polar versus non-polar milieux apparently can tell us quite a lot about the process of translocation.