ABSTRACT
Rhodopsin molecules are photochemically reactive membrane-embedded proteins, with seven transmembraneα-heliceswhichbindthechromophoreretinal(vitaminAaldehyde).1,2 Rhodopsins are classi©edintotwogroups,microbial(type1)andmammalian(type2).3 Type 2 rhodopsins, such as visual pigments,areG-protein-coupledreceptors(GPCRs)thatarewidespreadinvertebratesandinvertebrates.2,4,5 §is chapter reviews type 1 rhodopsins functioning as light-driven ion transporters or photosensoryreceptorsinmicroorganisms.§eyarewidespreadinthemicrobialworldinprokaryotes (bacteriaandarchaea)andineukaryotes(fungiandalgae).3,6,7 A striking characteristic of these photoactiveproteinsistheirwiderangeofseeminglydissimilarfunctions.Somearelight-driventransporters,suchastheprotonpumpbacteriorhodopsin(BR)andthechloridepumphalorhodopsin(HR) (Figure49.1).3,8 Others are light sensors, such as the phototaxis receptors sensory rhodopsins I and II (SRIandSRII).9 SRI and SRII relay signals by protein-protein interactions to integral membrane transducerproteinsHtrIandHtrII,respectively,andSRI-HtrIandSRII-HtrIIcomplexescontroltheÀagellar motorrotationthroughkinases(Figure49.1).§emicrobialrhodopsinshavebecomeafocusofinterest, inpartbecauseoftheirimportancetothegeneralunderstandingofionÀuxacrossmembranes,and communication between integral membrane proteins, about which little is known.
