Non-permanent proteins in membranes: when proteins come as visitors (Review)

The present review introduces the concept of 'non-permanent membrane proteins', to encompass the wide variety of proteins that are not found in a stable membrane-bound form under physiological conditions, yet they interact with the membrane at some stage of their specific course of action. Non-permanent membrane proteins can be codified by the cell's own genome, or else they may arise from a foreign genome. Non-permanent membrane proteins can be classified, according to the reversibility of the membrane interaction, into those with reversible and irreversible (very long-lived) membrane contacts. According to the nature (strength) of the interaction, non-permanent membrane proteins may be divided into those that interact weakly with the membrane, in an extrinsic-like form, and those that interact strongly with the membrane. The latter can in turn be classified into those that cause and those that do not cause covalent modification of the lipids, the latter behaving, after interacting with the membrane, in the way of the conventional intrinsic proteins. Multiple examples are provided for the different groups of non-permanent membrane proteins, and a more detailed description is given of three of them, representative of different groups, namely TrwD from plasmid R388, E. coli alpha-haemolysin and B. cereus sphingomyelinase.