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 pr oteins, 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 proto n transport may be reversed under appropriate conditions, Second, in B R-D85T as well as in the homologous halorhodopsin, protons and chlorid e ions compete for transport, e.g. the same molecule may transport eit her a positive or a negative ion. To rationalize these results, we pro pose a general model for ion translocation by bacterial rhodopsins whi ch 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 an d cytoplasmic accessibility, respectively, but change in accessibility (called the ''switch'') is a time dependent process in the millisecon d time range, A light-induced change of the isomerization state induce s ty of the assumptions. First, the isomerization state of the retinyl idene moiety governs the accessibility processes are kinetically indep endent, e,g, that relative rate constants in a given molecule determin e which process occurs first, ultimately defining the vectoriality of active transport.