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Bacteriorhodopsin. Proton efflux occurs via a transient linear water-molecule chain in a hydrophobic section of the Brho channel between Asp96 and Asp85 (Freier et al., 2011).  It can be converted to a chloride uptake pump by a single amino acid substitution at position 85.  However, halorhodopsin (3.E.1.2.1), which pumps chloride ions (Cl-) into the cell, apparently does not use hydrogen-bonded water molecules for Cl- transport (Muroda et al. 2012).  Nango et al. 2016 used time-resolved serial femtosecond crystallography and an x-ray free electron laser to visualize conformational changes in bRho from nanoseconds to milliseconds following photoactivation. An initially twisted retinal chromophore displaces a conserved tryptophan residue of transmembrane helix F on the cytoplasmic side of the protein while dislodging a key water molecule on the extracellular side. The resulting cascade of structural changes throughout the protein shows how motions are choreographed as bRho transports protons uphill against a transmembrane concentration gradient. Nango et al. 2016 have created a 3-d movie of structural changes in the protein showing that an initially twisted retinal chromophore displaces a conserved tryptophan residue of transmembrane helix F on the cytoplasmic side of the protein while dislodging a key water molecule on the extracellular side. Brho has light-independent lipid scramblase activity (Verchère et al. 2017). This activity occurs  at a rate >10,000 per trimer per second, comparable to that of other scramblases including bovine rhodopsin and fungal TMEM16 proteins. BR scrambles fluorescent analogues of common phospholipids but does not transport a glycosylated diphosphate isoprenoid lipid. In silico analyses suggested that membrane-exposed polar residues in transmembrane helices 1 and 2 of BR may provide the molecular basis for lipid translocation by coordinating the polar head-groups of transiting phospholipids. Consistent with this possibility, extensive coarse-grained molecular dynamics simulations of a BR trimer in a phospholipid membrane revealed water penetration along transmembrane helix 1 with the cooperation of a polar residue (Y147 in transmembrane helix 5) in the adjacent protomer. These findings suggest that the lipid translocation pathway may lie at or near the interface of the protomers of the BR trimer (Verchère et al. 2017). Retinal isomerization has been observed in the using a femtosecond x-ray laser (Nogly et al. 2018). S-TGA-1, a halobacterium-derived glycolipid, has the highest specificity to bRho, with a nanomolar dissociation constant (Inada et al. 2019). Weinert et al. 2019 recorded the structural changes in bacteriorhodopsin over 200 milliseconds in time. The snapshot from the first 5 milliseconds after photoactivation shows structural changes associated with proton release. From 10 to 15 milliseconds onwards, large additional structural rearrangements, up to 9 Å on the cytoplasmic side. Rotation of leucine-93 and phenylalanine-219 opens a hydrophobic barrier, leading to the formation of a water chain connecting the intracellular aspartic acid-96 with the retinal Schiff base. The formation of this proton wire recharges the membrane pump with a proton for the next cycle (Weinert et al. 2019).  The effect of membrane composition on the orientation and activity of bR has been reported (Palanco et al. 2017).  Efficient transfer of bRho from native membranes to covalently circularized nanodiscs has been accomplished (Yeh et al. 2018). The oligomeric status of BRho plays a role in the photocycle associated with short-range processes, such as retinal isomerization and deprotonation of the protonated Schiff base at the retinal pocket (Kao et al. 2019). Functional bacteriorhodopsin is efficiently solubilized and delivered to membranes by the chaperonin, GroEL (Deaton et al. 2004). Thirty one unfolded bR states in the presence of the retinal chromophore have been identified during denaturation (Jacobson et al. 2020).

Accession Number:P02945
Protein Name:BACR aka BOP aka VNG1467G
Molecular Weight:28256.00
Species:Halobacterium halobium [2242]
Number of TMSs:7
Location1 / Topology2 / Orientation3: Cell membrane1 / Multi-pass membrane protein2
Substrate H+

Cross database links:

RefSeq: NP_280292.1   
Entrez Gene ID: 1448071   
Pfam: PF01036   
BioCyc: HSP64091:VNG1467G-MONOMER   
KEGG: hal:VNG1467G   

Gene Ontology

GO:0016021 C:integral to membrane
GO:0005886 C:plasma membrane
GO:0005216 F:ion channel activity
GO:0009881 F:photoreceptor activity
GO:0007602 P:phototransduction
GO:0018298 P:protein-chromophore linkage
GO:0015992 P:proton transport

References (25)

[1] “The bacteriorhodopsin gene.”  Dunn   12049093
[2] “Studies on the light-transducing pigment bacteriorhodopsin.”  Dunn   6327180
[3] “Bacterioopsin, haloopsin, and sensory opsin I of the halobacterial isolate Halobacterium sp. strain SG1: three new members of a growing family.”  Soppa   8478333
[4] “Bacteriorhodopsin precursor. Characterization and its integration into the purple membrane.”  Seehra   6706999
[5] “Genome sequence of Halobacterium species NRC-1.”  Ng   11016950
[6] “Amino acid sequence of bacteriorhodopsin.”  Khorana   291920
[7] “Mass spectrometric analysis of integral membrane proteins: application to complete mapping of bacteriorhodopsins and rhodopsin.”  Ball   9541408
[8] “Attachment site(s) of retinal in bacteriorhodopsin.”  Katre   6794028
[9] “Electrospray-ionization mass spectrometry of intact intrinsic membrane proteins.”  Whitelegge   9655347
[10] “Bicelle crystallization: a new method for crystallizing membrane proteins yields a monomeric bacteriorhodopsin structure.”  Faham   11829498
[11] “Tertiary structure of bacteriorhodopsin. Positions and orientations of helices A and B in the structural map determined by neutron diffraction.”  Popot   2614846
[12] “Model for the structure of bacteriorhodopsin based on high-resolution electron cryo-microscopy.”  Henderson   2359127
[13] “Electron-crystallographic refinement of the structure of bacteriorhodopsin.”  Grigorieff   8676377
[14] “Three-dimensional structure of proteolytic fragment 163-231 of bacterioopsin determined from nuclear magnetic resonance data in solution.”  Barsukov   1606953
[15] “1H-15N-NMR studies of bacteriorhodopsin Halobacterium halobium. Conformational dynamics of the four-helical bundle.”  Orekhov   1332860
[16] “Surface of bacteriorhodopsin revealed by high-resolution electron crystallography.”  Kimura   9296502
[17] “X-ray structure of bacteriorhodopsin at 2.5-A from microcrystals grown in lipidic cubic phases.”  Pebay-Peyroula   9287223
[18] “Lipid patches in membrane protein oligomers: crystal structure of the bacteriorhodopsin-lipid complex.”  Essen   9751724
[19] “Proton transfer pathways in bacteriorhodopsin at 2.3 Angstrom resolution.”  Luecke   9632391
[20] “Structure of bacteriorhodopsin at 1.55-A resolution.”  Luecke   10452895
[21] “High-resolution X-ray structure of an early intermediate in the bacteriorhodopsin photocycle.”  Edman   10548112
[22] “Protein, lipid and water organization in bacteriorhodopsin crystals: a molecular view of the purple membrane at 1.9 A resolution.”  Belrhali   10467143
[23] “Coupling photoisomerization of retinal to directional transport in bacteriorhodopsin.”  Luecke   10903866
[24] “Helix deformation is coupled to vectorial proton transport in the photocycle of bacteriorhodopsin.”  Royant   10949307
[25] “Molecular mechanism of vectorial proton translocation by bacteriorhodopsin.”  Subramaniam   10949309
1AP9   1AT9   1BAC   1BAD   1BCT   1BHA   1BHB   1BM1   1BRD   1BRR   [...more]

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