Hydrophobic pockets at the membrane interface: an original mechanism for membrane protein interactions

Bridging Carotenoid-to-Bacteriochlorophyll Energy Transfer of Purple Bacteria LH2 With Temperature Variations: Insights From Conformational Changes

Photosynthesis is a key process for converting light energy into chemical energy and providing food for lives on Earth. Understanding the mechanism for the energy transfers could provide insights into regulating energy transfers in photosynthesis and designing artificial photosynthesis systems. Many efforts have been devoted to exploring the mechanism of temperature variations affecting the excitonic properties of LH2. In this study, we performed all-atom molecular dynamics (MD) simulations and quantum mechanics calculations for LH2 complex from purple bacteria along with its membrane environment under three typical temperatures: 270, 300, and 330 K. The structural analysis from validated MD simulations showed that the higher temperature impaired interactions at N-terminus of both α and β polypeptide helices and led to the dissociation of this hetero polypeptide dimer. Rhodopin-β-D-glucosides (RG1) moved centripetally with α polypeptide helices when temperature increased and enlarged their distances with bacteriochlorophylls molecules that have the absorption peak at 850 nm (B850), which resulted in reducing the coupling strengths between RG1 and B850 molecules. The present study reported a cascading mechanism for temperature regulating the energy transfers in LH2 of purple bacteria.

C-terminal cleavage of the LH1 α-polypeptide in the Sr2+-cultured Thermochromatium tepidum

The light-harvesting 1 reaction center (LH1-RC) complex in the thermophilic purple sulfur bacterium Thermochromatium (Tch.) tepidum binds Ca ions as cofactors, and Ca-binding is largely involved in its characteristic Q _y absorption at 915 nm and enhanced thermostability. Ca²⁺ can be biosynthetically replaced by Sr²⁺ in growing cultures of Tch. tepidum. However, the resulting Sr²⁺-substituted LH1-RC complexes in such cells do not display the absorption maximum and thermostability of those from Ca²⁺-grown cells, signaling that inherent structural differences exist in the LH1 complexes between the Ca²⁺- and Sr²⁺-cultured cells. In this study, we examined the effects of the biosynthetic Sr²⁺-substitution and limited proteolysis on the spectral properties and thermostability of the Tch. tepidum LH1-RC complex. Preferential truncation of two consecutive, positively charged Lys residues at the C-terminus of the LH1 α-polypeptide was observed for the Sr²⁺-cultured cells. A proportion of the truncated LH1 α-polypeptide increased during repeated subculturing in the Sr²⁺-substituted medium. This result suggests that the truncation is a biochemical adaptation to reduce the electrostatic interactions and/or steric repulsion at the C-terminus when Sr²⁺ substitutes for Ca²⁺ in the LH1 complex. Limited proteolysis of the native Ca²⁺-LH1 complex with lysyl protease revealed selective truncations at the Lys residues in both C- and N-terminal extensions of the α- and β-polypeptides. The spectral properties and thermostability of the partially digested native LH1-RC complexes were similar to those of the biosynthetically Sr²⁺-substituted LH1-RC complexes in their Ca²⁺-bound forms. Based on these findings, we propose that the C-terminal domain of the LH1 α-polypeptide plays important roles in retaining proper structure and function of the LH1-RC complex in Tch. tepidum.

Functional competition within a membrane: Lipid recognition vs. transmembrane helix oligomerization

Binding of specific lipids to large, polytopic membrane proteins is well described, and it is clear that such lipids are crucial for protein stability and activity. In contrast, binding of defined lipid species to individual transmembrane helices and regulation of transmembrane helix monomer-oligomer equilibria by binding of distinct lipids is a concept, which has emerged only lately. Lipids bind to single-span membrane proteins, both in the juxta-membrane region as well as in the hydrophobic membrane core. While some interactions counteract transmembrane helix oligomerization, in other cases lipid binding appears to enhance oligomerization. As reversible oligomerization is involved in activation of many membrane proteins, binding of defined lipids to single-span transmembrane proteins might be a mechanism to regulate and/or fine-tune the protein activity. But how could lipid binding trigger the activity of a protein? How can binding of a single lipid molecule to a transmembrane helix affect the structure of a transmembrane helix oligomer, and consequently its signaling state? These questions are discussed in the present article based on recent results obtained with simple, single-span transmembrane proteins. This article is part of a Special Issue entitled: Lipid-protein interactions.

Determinants of aquaporin-4 assembly in orthogonal arrays revealed by live-cell single-molecule fluorescence imaging

We investigated the molecular determinants of aquaporin-4 (AQP4) assembly in orthogonal arrays of particles (OAPs) by visualizing fluorescently labeled AQP4 mutants in cell membranes using quantum-dot single-particle tracking and total internal reflection fluorescence microscopy. The full-length ;long' (M1) form of AQP4 diffused freely in membranes and did not form OAPs, whereas the ;short' (M23) form of AQP4 formed OAPs and was nearly immobile. Analysis of AQP4 deletion mutants revealed progressive disruption of OAPs by the addition of three to seven residues at the AQP4-M23 N-terminus, with polyalanines as effective as native AQP4 fragments. OAPs disappeared upon downstream deletions of AQP4-M23, which, from analysis of point mutants, involves N-terminus interactions of residues Val24, Ala25 and Phe26. OAP formation was also prevented by introducing proline residues at sites just downstream from the hydrophobic N-terminus of AQP4-M23. AQP1, an AQP4 homolog that does not form OAPs, was induced to form OAPs upon replacement of its N-terminal domain with that of AQP4-M23. Our results indicate that OAP formation by AQP4-M23 is stabilized by hydrophobic intermolecular interactions involving N-terminus residues, and that absence of OAPs in AQP4-M1 results from non-selective blocking of this interaction by seven residues just upstream from Met23.

Calcium ions are required for the enhanced thermal stability of the light-harvesting-reaction center core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum

Thermochromatium tepidum is a thermophilic purple sulfur photosynthetic bacterium collected from the Mammoth Hot Springs, Yellowstone National Park. A previous study showed that the light-harvesting-reaction center core complex (LH1-RC) purified from this bacterium is highly stable at room temperature (Suzuki, H., Hirano, Y., Kimura, Y., Takaichi, S., Kobayashi, M., Miki, K., and Wang, Z.-Y. (2007) Biochim. Biophys. Acta 1767, 1057-1063). In this work, we demonstrate that thermal stability of the Tch. tepidum LH1-RC is much higher than that of its mesophilic counterparts, and the enhanced thermal stability requires Ca2+ as a cofactor. Removal of the Ca2+ from Tch. tepidum LH1-RC resulted in a complex with the same degree of thermal stability as that of the LH1-RCs purified from mesophilic bacteria. The enhanced thermal stability can be restored by addition of Ca2+ to the Ca2+-depleted LH1-RC, and this process is fully reversible. Interchange of the thermal stability between the two forms is accompanied by a shift of the LH1 Qy transition between 915 nm for the native and 880 nm for the Ca2+-depleted LH1-RC. Differential scanning calorimetry measurements reveal that degradation temperature of the native LH1-RC is 15 degrees C higher and the enthalpy change is about 28% larger than the Ca2+-depleted LH1-RC. Substitution of the Ca2+ with other metal cations caused a decrease in thermal stability of an extent depending on the properties of the cations. These results indicate that Ca2+ ions play a dual role in stabilizing the structure of the pigment-membrane protein complex and in altering its spectroscopic properties, and hence provide insight into the adaptive strategy of this photosynthetic organism to survive in extreme environments using natural resources.

Thermodynamics of the beta(2) association in light-harvesting complex I of Rhodospirillum rubrum. Implication of peptide identity in dimer stability

The core light-harvesting LH1 protein from Rhodospirillum rubrum can dissociate reversibly in the presence of n-octyl-beta-D-glucopyranoside into smaller subunit forms, exhibiting a dramatic blue-shift in absorption. During this process, two main species are observed: a dimer that absorbs at 820 nm (B820) and a monomer absorbing at 777 nm (B777). In the presence of n-octyl-beta-D-glucopyranoside, we have previously shown that the B820 form is not only constituted by the alphabeta heterodimer alone, but that it exists in an equilibrium between the alphabeta heterodimer and beta(2) homodimer states. We investigated the dissociation equilibrium for both oligomeric B820 forms. Using a theoretical model for alphabeta and beta(2), we conclude that the B820 homodimer is stabilized by both hydrophobic effects (entropy) and non-covalent bonds (enthalpy). We discuss a possible interpretation of the energy changes.

Secretin receptor oligomers form intracellularly during maturation through receptor core domains

Oligomerization of numerous G protein-coupled receptors has been documented, including the prototypic family B secretin receptor. The clinical significance of oligomerization of this receptor became clear with the recent observation that a misspliced form present in pancreatic cancer could associate with the wild-type receptor and act as a dominant negative inhibitor of its normal growth inhibitory function. Our goal was to explore the molecular mechanism of this interaction using bioluminescence (BRET) and fluorescence (FRET) resonance energy transfer and fluorescence microscopy with a variety of receptor constructs tagged with luciferase or cyan or yellow fluorescent proteins. BRET signals comparable to those obtained from cells coexpressing differentially tagged wild-type receptors were observed for similarly tagged secretin receptors in which all or part of the amino-terminal domain was deleted. As expected, neither of these constructs bound secretin, and only the partially truncated construct sorted to the plasma membrane. Receptors lacking the majority of the carboxyl-terminal domain, including that important for phosphorylation-mediated desensitization, also produced BRET signals above background. These findings suggested that the receptor's membrane-spanning core is responsible for secretin receptor oligomerization. Interestingly, alanine substitutions for a -GxxxG- helix interaction motif in transmembrane segment 7 created nonfunctional receptors that were capable of forming oligomers. Furthermore, treatment of receptor-expressing cells with brefeldin A did not eliminate the BRET signals, and morphologic FRET experiments confirmed the expected subcellular localizations of receptor oligomers. We conclude that secretin receptor oligomerization occurs through -GxxxG- motif-independent interactions of transmembrane segments during the maturation of nascent molecules.

Solution Structures of the Core Light-harvesting α and β Polypeptides from Rhodospirillum rubrum: Implications for the Pigment–Protein and Protein–Protein Interactions

We have determined the solution structures of the core light-harvesting (LH1) alpha and beta-polypeptides from wild-type purple photosynthetic bacterium Rhodospirillum rubrum using multidimensional NMR spectroscopy. The two polypeptides form stable alpha helices in organic solution. The structure of alpha-polypeptide consists of a long helix of 32 amino acid residues over the central transmembrane domain and a short helical segment at the N terminus that is followed by a three-residue loop. Pigment-coordinating histidine residue (His29) in the alpha-polypeptide is located near the middle of the central helix. The structure of beta-polypeptide shows a single helix of 32 amino acid residues in the membrane-spanning region with the pigment-coordinating histidine residue (His38) at a position close to the C-terminal end of the helix. Strong hydrogen bonds have been identified for the backbone amide protons over the central helical regions, indicating a rigid property of the two polypeptides. The overall structures of the R.rubrum LH1 alpha and beta-polypeptides are different from those previously reported for the LH1 beta-polypeptide of Rhodobacter sphaeroides, but are very similar to the structures of the corresponding LH2 alpha and beta-polypeptides determined by X-ray crystallography. A model constructed for the structural subunit (B820) of LH1 complex using the solution structures reveals several important features on the interactions between the LH1 alpha and beta-polypeptides. The significance of the N-terminal regions of the two polypeptides for stabilizing both B820 and LH1 complexes, as clarified by many experiments, may be attributed to the interactions between the short N-terminal helix (Trp2-Gln6) of alpha-polypeptide and a GxxxG motif in the beta-polypeptide.