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

The first study by nmr of the integral membrane protein, the bacterial ligh t-harvesting (LH) antenna protein LH1 beta, is reported. The photosynthetic apparatus of purple bacteria contains two different kinds of antenna compl exes (LH1 and LH2), which consist of two small integral membrane proteins a lpha and beta, each of approximately 6 kDa, and bacteriochlorophyll and car otenoid pigments. We have purified the antenna polypeptide LH1 beta from Rh odobacter sphaeroides, and have recorded CD spectra and a series of two-dim ensional nmr spectra. A comparison of CD spectra of LH1 beta observed in or ganic solvents and detergent micelles shows that the helical character of t he peptide does not change appreciably between the two milieus. A significa ntly high-field shifted methyl signal was observed both in organic solvents and in detergent micelles, implying that a similar three-dimensional struc ture is present in each case. However, the H-1-nmr signals observed in orga nic solvents had a narrower line width and better resolution, and it is sho wn that in this case organic solvents provide a better medium for nmr studi es than detergent micelles. A sequential assignment has been carried out on the C-terminal transmembrane region, which is the region in which the pigm ent is bound. The region is shown to have a helical structure by the chemic al shift values of the alpha-CH protons and the presence of nuclear Overhau ser effects characteristic of helices. An analysis of the amide proton chem ical shifts of the residues surrounding the histidine chlorophyll ligand su ggests that the local structure is well ordered even in the absence of prot ein-lipid and protein-pigment interactions. Its structure was determined fr om 348 nmr-derived constraints by using distance geometry calculations. The polypeptide contains an alpha-helix extending from Leu19 (position of cyto plasmic surface) to Trp44 (position of periplasmic surface). The helix is b ent, as expected from the amide proton chemical shifts, and it is similar t o the polypeptide fold of the previously determined crystal structure of Rh odopseudomonas acidophila Ac10050 LH2 beta (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 c omplex. (C) 1999 John Wiley & Sons, Inc.