General concept fur ion translocation by halobacterial retinal proteins: The isomerization/switch/transfer (IST) model

Bacteriorhodopsin (BR), which transports protons out of the cell in a light-driven process, is one of the best-studied energy-transducing proteins, However, a consensus on the exact molecular mechanism has not been reached. Matters are complicated by two experimental facts. First, recent results using BR mutants (BR-D85T) and the homologous protein sensory rhodopsin I demonstrate that the vectoriality of active proton transport may be reversed under appropriate conditions, Second, in BR-D85T as well as in the homologous halorhodopsin, protons and chloride ions compete for transport, e.g. the same molecule may transport either a positive or a negative ion. To rationalize these results, we propose a general model for ion translocation by bacterial rhodopsins which is mainly based on two Schiff base in the protein; e.g. all-trans, 15-anti, and 13-cis-15-anti direct the Schiff base to extracellular and cytoplasmic accessibility, respectively, but change in accessibility (called the ''switch'') is a time dependent process in the millisecond time range, A light-induced change of the isomerization state induces ty of the assumptions. First, the isomerization state of the retinylidene moiety governs the accessibility processes are kinetically independent, e,g, that relative rate constants in a given molecule determine which process occurs first, ultimately defining the vectoriality of active transport.