Effects of Mutations of Lys41 and Asp102 of Bacteriorhodopsin

Two substates in the O intermediate of the light-driven proton pump archaerhodopsin-2

The proton pumping cycle of archaerhodopsin-2 (aR2) was investigated over a wide pH range and at different salt concentrations. We have found that two substates, which are spectroscopically and kinetically distinguishable, occur in the O intermediate. The first O-intermediate (O1) absorbs maximumly at ~580 nm, whereas the late O-intermediate (O2) absorbs maximumly at 605 nm. At neutral pH, O1 is in rapid equilibrium with the N intermediate. When the medium pH is increased, O1 becomes less stable than N and, in proportion to the amount of O1 in the dynamic equilibrium between N and O1, the formation rate of O2 decreases. By contrast, the decay rate of O2 increases ~100 folds when the pH of a low-salt membrane suspension is increased from 5.5 to 7.5 or when the salt concentration is increased to 2 M KCl. Together with our recent study on two substates in the O intermediate of bacteriorhodopsin (bR), the present study suggests that the thermally activated re-isomerization of the retinylidene chromophore into the initial all-trans configuration takes place in the O1-to-O2 transition; that is, O1 contains a distorted 13-cis chromophore. It is also found that the pKa value of the key ionizable residue (Asp101^aR2, Asp96^bR) in the proton uptake channel is elevated in the O1 state of aR2 as compared to the O1 state of bR. This implies that the structural property of O1 in the aR2 photocycle can be investigated over a wide pH range.

Two Consecutive Polar Amino Acids at the End of Helix E are Important for Fast Turnover of the Archaerhodopsin Photocycle

Archaerhodopsins (ARs) is one of the members of microbial rhodopsins. Threonine 164 (T164) and serine 165 (S165) residues of the AR from Halorubrum sp. ejinoor (HeAR) are fully conserved in ARs, although they are far from the proton transfer channel and the retinal Schiff base, and are likely involved in a hydrogen-bonding network at the end of the Helix E where most microbial rhodopsins assume a "bent structure". In the present work, T164 and/or S165 were replaced with an alanine (A), and the photocycles of the mutants were analyzed with flash photolysis. The amino acid replacements caused profound changes to the photocycle of HeAR including prolonged photocycle, accelerated decay of M intermediate and appearance of additional two intermediates which were evident in T164A- and T164A/S165A-HeAR photocyles. These results suggest that although T164 and S165 are located at the far end of the photoactive center, these two amino acid residues are important for maintaining the fast turnover of the HeAR photocycle. The underlying molecular mechanisms are discussed in relation to hydrogen-bonding networks involving these two amino acids. Present study may arouse our interests to explore the functional role of the well-conserved "bent structure" in different types of microbial rhodopsin.

Cell-material interactions revealed via material techniques of surface patterning

Cell-material interactions constitute a key fundamental topic in biomaterials study. Various cell cues and matrix cues as well as soluble factors regulate cell behaviors on materials. These factors are coupled with each other as usual, and thus it is very difficult to unambiguously elucidate the role of each regulator. The recently developed material techniques of surface patterning afford unique ways to reveal the underlying science. This paper reviews the pertinent material techniques to fabricate patterns of microscale and nanoscale resolutions, and corresponding cell studies. Some issues are emphasized, such as cell localization on patterned surfaces of chemical contrast, and effects of cell shape, cell size, cell-cell contact, and seeding density on differentiation of stem cells. Material cues to regulate cell adhesion, cell differentiation and other cell events are further summed up. Effects of some physical properties, such as surface topography and matrix stiffness, on cell behaviors are also discussed; nanoscaled features of substrate surfaces to regulate cell fate are summarized as well. The pertinent work sheds new insight into the cell-material interactions, and is stimulating for biomaterial design in regenerative medicine, tissue engineering, and high-throughput detection, diagnosis, and drug screening.