Subdomains in the F and G helices of bacteriorhodopsin regulate the conformational transitions of the reprotonation mechanism

G-protein-coupled receptor domain overexpression in Halobacterium salinarum: long-range transmembrane interactions in heptahelical membrane proteins

The aminergic alpha(2b)-adrenergic receptor (alpha(2b)-AR) third intracellular loop (alpha(2b)-AR 3i) mediates receptor subcellular compartmentalization and signal transduction processes via ligand-dependent interaction with G(i)- and G(o)- proteins. To understand the structural origins of these processes we engineered several lengths of alpha(2b)-AR 3i into the third intracellular loop of the proton pump bacteriorhodopsin (bR) and produced the fusion proteins in quantities suitable for physical studies. The fusion proteins were expressed in the Archaeon Halobacterium salinarum and purified. A highly expressed fusion protein was crystallized from bicelles and diffracted to low resolution on an in-house diffractometer. The bR-alpha(2b)-AR 3i(203-292) protein possessed a photocycle slightly perturbed from that of the wild-type bR. The first half of the fusion protein photocycle, correlated with proton release, is accelerated by a factor of 3, whereas the second half, correlated with proton uptake, is slightly slower than wild-type bR. In addition, there is a large decrease in the pK(a), (from 9.6 to 8.3) of the terminal proton release group in the unphotolyzed state of bR-alpha(2b)-AR 3i as deduced from the pH-dependence of the M-formation. Perturbation of a cytoplasmic loop has thus resulted in the perturbation of proton release at the extracellular surface. The current work indicates that long-range and highly coupled intramolecular interactions exist that are capable of "transducing" structural perturbations (e.g., signals) across the cellular membrane. This gene fusion approach may have general applicability for physical studies of G-protein-coupled receptor domains in the context of the bR structural scaffold.

Conformational heterogeneity of transmembrane residues after the Schiff base reprotonation of bacteriorhodopsin

bR, N-like and O-like intermediate states of [15N]methionine-labelled wild type and D85N/T170C bacteriorhodopsin were accumulated in native membranes by controlling the pH of the preparations. 15N cross polarization and magic angle sample spinning (CPMAS) NMR spectroscopy allowed resolution of seven out of nine resonances in the bR-state. It was possible to assign some of the observed resonances by using 13C/15N rotational echo double resonance (REDOR) NMR and Mn2+ quenching as well as D2O exchange, which helps to identify conformational changes after the bacteriorhodopsin Schiff base reprotonation. The significant differences in chemical shifts and linewidths detected for some of the resonances in N- and O-like samples indicate changes in conformation, structural heterogeneity or altered molecular dynamics in parts of the protein.

Downstream coding region determinants of bacterio-opsin, muscarinic acetylcholine receptor and adrenergic receptor expression in Halobacterium salinarum

The aim of this work is to develop a prokaryotic system capable of expressing membrane-bound receptors in quantities suitable for biochemical and biophysical studies. Our strategy exploits the endogenous high-level expression of the membrane protein bacteriorhodopsin (BR) in the Archaeon Halobacterium salinarum. We attempted to express the human muscarinic acetylcholine (M(1)) and adrenergic (a2b) receptors by fusing the coding region of the m1 and a2b genes to nucleotide sequences known to direct bacterio-opsin (bop) gene transcription. The fusions included downstream modifications to produce non-native carboxyl-terminal amino acids useful for protein identification and purification. bop mRNA and BR accumulation were found to be tightly coupled and the carboxyl-terminal coding region modifications perturbed both. m1 and a2b mRNA levels were low, and accumulation was sensitive to both the extent of the bop gene fusion and the specific carboxyl-terminal coding sequence modifications included. Functional a2b adrenergic receptor expression was observed to be dependent on the downstream coding region. This work demonstrates that a critical determinant of expression resides in the downstream coding region of the wild-type bop gene and manipulation of the downstream coding region of heterologous genes may affect their potential for expression in H. salinarum.