COMPONENT ANALYSIS OF THE FAST PHOTOELECTRIC SIGNAL FROM MODEL BACTERIORHODOPSIN MEMBRANES .3. EFFECT OF THE POINT MUTATION ASPARTATE-212 -] ASPARAGINE-212

This report investigates the effect of mutation in bacteriorhodopsin ( BR), which changes residue 212 from aspartate to asparagine (D212N), o n the individual components of the fast photoelectric signal; these co mponents are generated by light-induced rapid charge displacement. D21 2N was expressed in Halobacterium halobium. The photoelectric data wer e subject to component analysis as outlined in the first of the two co mpanion papers. With the aid of equivalent circuit analysis, we identi fied two components: B1 and B2. The analysis shows that the B2 compone nt is absent in the medium to high pH range, while the B1 component is intact. The B2 component is, however, present at low pH but with a re versed polarity as compared with the B2 at neutral and high pH. The B2 component at low pH is also dependent on Cl(-)ntration. This low-pH s ignal may be generated either by interfacial Cl(-)ransfer or by a Cl-- denterfacial proton transfer. This low-pH signal, although less conspi cuous, is also observable in the wild-type BR. The contrasting respons es of the two components at the medium to high pH range, again support ing the validity of our component analysis.