Halorhodopsin: a light-driven active chloride transport system

Crystal structure of Halobacterium salinarum halorhodopsin with a partially depopulated primary chloride-binding site

The transmembrane pump halorhodopsin in halophilic archaea translocates chloride ions from the extracellular to the cytoplasmic side upon illumination. In the ground state a tightly bound chloride ion occupies the primary chloride-binding site (CBS I) close to the protonated Schiff base that links the retinal chromophore to the protein. The light-triggered trans-cis isomerization of retinal causes structural changes in the protein associated with movement of the chloride ion. In reverse, chemical depletion of CBS I in Natronomonas pharaonis halorhodopsin (NpHR) through deprotonation of the Schiff base results in conformational changes of the protein: a state thought to mimic late stages of the photocycle. Here, crystals of Halobacterium salinarum halorhodopsin (HsHR) were soaked at high pH to provoke deprotonation of the Schiff base and loss of chloride. The crystals changed colour from purple to yellow and the occupancy of CBS I was reduced from 1 to about 0.5. In contrast to NpHR, this chloride depletion did not cause substantial conformational changes in the protein. Nevertheless, two observations indicate that chloride depletion could eventually result in structural changes similar to those found in NpHR. Firstly, the partially chloride-depleted form of HsHR has increased normalized B factors in the region of helix C that is close to CBS I and changes its conformation in NpHR. Secondly, prolonged soaking of HsHR crystals at high pH resulted in loss of diffraction. In conclusion, the conformation of the chloride-free protein may not be compatible with this crystal form of HsHR despite a packing arrangement that hardly restrains helices E and F that presumably move during ion transport.

Halophiles: biology, adaptation, and their role in decontamination of hypersaline environments

The unique cellular enzymatic machinery of halophilic microbes allows them to thrive in extreme saline environments. That these microorganisms can prosper in hypersaline environments has been correlated with the elevated acidic amino acid content in their proteins, which increase the negative protein surface potential. Because these microorganisms effectively use hydrocarbons as their sole carbon and energy sources, they may prove to be valuable bioremediation agents for the treatment of saline effluents and hypersaline waters contaminated with toxic compounds that are resistant to degradation. This review highlights the various strategies adopted by halophiles to compensate for their saline surroundings and includes descriptions of recent studies that have used these microorganisms for bioremediation of environments contaminated by petroleum hydrocarbons. The known halotolerant dehalogenase-producing microbes, their dehalogenation mechanisms, and how their proteins are stabilized is also reviewed. In view of their robustness in saline environments, efforts to document their full potential regarding remediation of contaminated hypersaline ecosystems merits further exploration.

Chloride ATPase pumps in nature: do they exist?

Five widely documented mechanisms for chloride transport across biological membranes are known: anion-coupled antiport, Na+ and H(+)-coupled symport, Cl- channels and an electrochemical coupling process. These transport processes for chloride are either secondarily active or are driven by the electrochemical gradient for chloride. Until recently, the evidence in favour of a primary active transport mechanism for chloride has been inconclusive despite numerous reports of cellular Cl(-)-stimulated ATPases coexisting, in the same tissue, with uphill ATP-dependent chloride transport. Cl(-)-stimulated ATPase activity is a ubiquitous property of practically all cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase pump activity. Recent studies of Cl(-) -stimulated ATPase activity and ATP-dependent chloride transport in the same plasma membrane system, including liposomes, strongly suggest a mediation by the ATPase in the net movement of chloride up its electrochemical gradient across the plasma membrane structure. Contemporary evidence points to the existence of Cl(-)-ATPase pumps; however, these primary active transporters exist as either P-, F- or V-type ATPase pumps depending upon the tissue under study.

Cl(-)-ATPases: biological active transporters

Five widely documented mechanisms of chloride transport across plasma membranes are: anion-coupled antiport; sodium and hydrogen-coupled symport; Cl- channels; and an electrochemical coupling process. No genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular Cl(-)-stimulated ATPases co-existing, in the same tissue, with uphill chloride transport that could not be accounted for by the five common chloride transport processes. Cl(-)-stimulated ATPase activity is a common property of practically all biological cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase activity. Recent studies of Cl(-)-stimulated ATPase activity and active chloride transport in the same membrane system, including liposomes, suggest a mediation by the ATPase in net movement of chloride up its electrochemical gradient across plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl(-)-stimulated ATPases.

Cl(-)-ATPases: Novel primary active transporters in biology

Five widely documented mechanisms of chloride transport across plasma membranes are anion-coupled antiport, sodium and hydrogen-coupled symport, Cl(-)channels, and an electrochemical coupling process. No genetic evidence has yet been provided for primary active chloride transport despite numerous reports of cellular Cl(-)-stimulated ATPases co-existing, in the same tissue, with uphill chloride transport that could not be accounted for by the five common chloride transport processes. Cl(-)-stimulated ATPase activity is a common property of practically all biological cells with the major location being of mitochondrial origin. It also appears that plasma membranes are sites of Cl(-)-stimulated ATPase activity. Recent studies of Cl(-)-stimulated ATPase activity and active chloride transport in the same membrane system, including liposomes, suggest a medication by the ATPase in net movement of chloride up its electrochemical gradient across plasma membranes. Further studies, especially from a molecular biological perspective, are required to confirm a direct transport role to plasma membrane-localized Cl(-)-stimulated ATPases. J. Exp. Zool. 289:215-223, 2001.

Nano- and microsecond time-resolved FTIR spectroscopy of the halorhodopsin photocycle

Step-scan Fourier transform infrared spectroscopy with 50 ns time resolution was applied to the early stages of the photocycle of halorhodopsin (hR) for the temperature range 3-42 degrees C. Kinetic data analysis with global fitting revealed two distinct kinetic processes associated with relaxations of the early red-shifted photoproduct hK; these processes have time constants tau 1 approximately equal to 280 ns and tau 2 approximately equal to 360 microns at 20 degrees C. Spectral features demonstrate that the tau 1 process corresponds to a transition between two distinct bathointermediates, hKE and hKL. The vibrational difference bands associated with both tau 1 and tau 2 transitions are spread throughout the whole 1800-900 cm-1 range. However, the largest bands correspond to ethylenic C=C stretches, fingerprint C-C stretches and hydrogen out-of-plane (HOOP) wags of the retinal chromophore. The time evolution of these difference bands indicate that both the tau 1 and tau 2 decay processes involve principally a relaxation of the chromophore and its immediate environment. The decay of the intense HOOP vibrations is nearly equally divided between the tau 1 and tau 2 processes, indicating a complex chromophore relaxation from a twisted nonrelaxed conformation in the primary (hKE) bathointermediate, to a less-twisted structure in hKL, and finally to a roughly planar structure in the hypsochromically shifted hL intermediate. This conclusion is also supported by the unexpectedly large positive entropy of activation observed for the tau 1 process. The two relaxations from hKE to hL are largely analogous to corresponding relaxations (KE-->KL-->L) in the bacteriorhodopsin photocycle, except that the second step is slowed down by over 200-fold in hR.