A light-harvesting antenna protein retains its folded conformation in the absence of protein-lipid and protein-pigment interactions

Jun Kikuchi, Tetsuo Asakura*, Paul A Loach, Pamela S Parkes-Loach, Keizo Shimada, C. Neil Hunter, Matthew J. Conroy, Michael P. Williamson

*Corresponding author for this work

Research output: Contribution to journalArticle

18 Scopus citations

Abstract

The first study by nmr of the integral membrane protein, the bacterial light-harvesting (LH) antenna protein LH1β, is reported. The photosynthetic apparatus of purple bacteria contains two different kinds of antenna complexes (LH1 and LH2), which consist of two small integral membrane proteins α and β, each of approximately 6 kDa, and bacteriochlorophyll and carotenoid pigments. We have purified the antenna polypeptide LH1β from Rhodobacter sphaeroides, and have recorded CD spectra and a series of two- dimensional nmr spectra. A comparison of CD spectra of LH1β observed in organic solvents and detergent micelles shows that the helical character of the peptide does not change appreciably between the two milieus. A significantly high-field shifted methyl signal was observed both in organic solvents and in detergent micelles, implying that a similar three-dimensional structure is present in each case. However, the 1H-nmr signals observed in organic solvents had a narrower line width and better resolution, and it is shown that in this case organic solvents provide a better medium for nmr studies than detergent micelles. A sequential assignment has been carried out on the C-terminal transmembrane region, which is the region in which the pigment is bound. The region is shown to have a helical structure by the chemical shift values of the α-CH protons and the presence of nuclear Overhauser effects characteristic of helices. An analysis of the amide proton chemical shifts of the residues surrounding the histidine chlorophyll ligand suggests that the local structure is well ordered even in the absence of protein-lipid and protein-pigment interactions. Its structure was determined from 348 nmr-derived constraints by using distance geometry calculations. The polypeptide contains an α-helix extending from Leu19 (position of cytoplasmic surface) to Trp44 (position of periplasmic surface). The helix is bent, as expected from the amide proton chemical shifts, and it is similar to the polypeptide fold of the previously determined crystal structure of Rhodopseudomonas acidophila Ac10050 LH2β (S. M. Prince et al., Journal of Molecular Biology, 1997, Vol. 268, pp. 412-423). It is concluded that the polypeptide conformation of this region may facilitate assembly of the LH complex.

Original languageEnglish (US)
Pages (from-to)361-372
Number of pages12
JournalBiopolymers
Volume49
Issue number5
DOIs
StatePublished - Apr 15 1999

Keywords

  • Light-harvesting antenna complex
  • Membrane protein
  • Nuclear Overhauser enhancement
  • nmr

ASJC Scopus subject areas

  • Biophysics
  • Biochemistry
  • Biomaterials
  • Organic Chemistry

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