Color-changing mutation in the E-F loop of proteorhodopsin

The replacement of Ala178 with Arg in the E-F loop of proteorhodopsin (PR), an archaeal-type rhodopsin in marine bacteria which shifts the [[lambda].sub.max] from 525 to 545 nm at neutral pH was examined to understand the molecular mechanism of the unusual mutation effect on color tuning. The results provide insight into unique structure of E-F loop in PR and the observed changes in color and p[K.sub.a] due to the mutation of Ala178 that disrupted the structure in the transmembrane region.