Isolation, Size Estimates, and Spectral Heterogeneity of an Oligomeric Series of Light-Harvesting 1 Complexes from Rhodobacter sphaeroides

A series of light-harvesting 1 (LH1) complexes was isolated by lithium dodecyl sulfate−polyacrylamide gel electrophoresis at 4 °C from Rhodobacter sphaeroides M21, which lacks the peripheral light-harvesting 2 (LH2) complex. This ladder of LH1 bands was also demonstrated in the wild type, partially superimposed upon a smaller number of LH2 complexes. An assessment of electrophoretic mobility vs acrylamide concentration, in which the reaction center LM particle and annular LH1 and LH2 complexes were used as standards of known structure, indicated that the LH1 gel bands 2 to 10 represent regular oligomers of an αβ heterodimeric unit, that vary in size from (αβ)2 - 3 to (αβ)10 - 11. The isolated LH1 complexes exhibited oligomeric state dependent optical properties, characterized by red shifts in near-IR absorption and emission maxima at 77 K of ∼6 nm as aggregate sizes increased from ∼3 to 7−8 αβ-heterodimers, accompanied by shifts in highly polarized fluorescence from the blue to the red side of the absorption band. This has been explained by the oligomerization of heterodimers to form a curvilinear array of excitonically coupled chromophores, with the anisotropic long-wavelength component, designated originally as B896, corresponding to low energy excitonic transitions arising from interactions within inhomogeneous BChl clusters [Westerhuis et al. (1999) J. Phys. Chem. B 103, 7733−7742]. Differences in electrophoretic profiles of LH1 bands between strains M21 and M2192, an LH1-only strain that also lacks PufX, further suggested that the more rapidly migrating bands represent arced fragments of the curvilinear array of LH1 complexes thought to exist as a large closed circular structure only in the latter strain. The electrophoretic banding pattern also indicated that the LH1 complex may be located at the peripheries of dimeric intramembrane particle arrays seen in freeze-fracture replicas of tubular M21 membranes; the possible role for the PufX protein in the assembly of these structures is discussed.