Trapping of an Assembly Intermediate of Photosynthetic LH1 Antenna beyond B820 Subunit

Abstract

Most photosynthetic LH1 antennae undergo dissociation into B820 subunits, suggesting their universal character as structural modules. However, dissociation into subunits seems to occur reversibly only in the absence of carotenoids and the subunits were never found to bind carotenoids. The interactions of carotenoids with B820 have been studied in a newly developed reconstitution assay of the LH1 antenna from Rhodospirillum rubrum (Fiedor, L., Akahane, J., and Koyama, Y. (2004) Biochemistry 43, 16487-16496). These model studies show that B820 subunits strongly interact with carotenoids and spontaneously form stable LH1-like complexes with substoichiometric carotenoid content. This is the first experimental evidence that B820 may occur as a short-lived intermediate in the assembly of LH1 in vivo. The resulting complex of B820 subunits with carotenoid, termed iB873, is homogeneous, according to ion exchange chromatography and reproducible pigment composition. The iB873-bound carotenoid is as efficient in energy transfer to bacteriochlorophyll as the one in native antenna. To our knowledge, iB873 is the first complex binding functional carotenoid, with the spectral and biochemical properties intermediate between that of B820 and the fully assembled LH1.The progress in biochemical (1-3), spectroscopic (4 -8), and structural characterization (9 -16) of purple bacterial lightharvesting complexes has greatly advanced our understanding of photosynthesis. These intramembranous antennae are oligomers of small, heterodimeric proteins carrying BChls 1 and Crts. In peripheral light-harvesting complexes, LH2, LH3, or LH4, 8 or 9 such units assemble to a near perfect circular structure (9 -11). The core light-harvesting complex, LH1, is an oligomer of similar units, but here the size and the shape seem to be more flexible (9 -11,17).To understand the assembly of the pigment-protein complexes, a detailed knowledge is necessary about the role of each particular component in the assembly, and of their interactions in the oligomeric state. The oligomeric character and modularity of antenna structures became already apparent before the high resolution structural data were obtained, mainly because of the isolation of a B820 subunit form of LH1 (2,18,19). Virtually all LH1 complexes were found to undergo dissociation into such subunits, suggesting their function as a universal structural motif (2). They are thought to consist of a dimeric form of BChl, non-covalently attached to two short hydrophobic polypeptides ␣ and ␤, organized into a basic unit ␣␤⅐(BChla) 2 or its dimer (20 -22). B820, however, has mostly been obtained from Crt-less LH1, and only here is the process fully reversible: depending on the detergent concentration, B820 can be reversibly oligomerized in vitro to Crt-less LH1 (low detergent concentration) or dissociated into smaller subunits that still contain monomeric BChl (high detergent concentration) (18,20,21,23). If B820 is prepared by dissociation of Crt-containing LH1, the Crts are lost in the process. In this c...

Probing binding site of bacteriochlorophyll a and carotenoid in the reconstituted LH1 complex from Rhodospirillum rubrum S1 by Stark spectroscopy

“…Recently, reconstitutions were reported into the LH1 complex from Rs. rubrum using a range of well-defined carotenoids Fiedor et al 2004;Fiedor and Scheer 2005). These studies used a series of purified carotenoids with different extents of p-conjugation.…”

Strategies to enhance the excitation energy-transfer efficiency in a light-harvesting system using the intra-molecular charge transfer character of carotenoids

“…rubrum G9+ according to the reported protocols 56,57 . The B820 heterodimers were isolated from the native LH1 complex as described previously 32,58,59 following solubilization with 1.2% octyl-β-glucopyranoside (β-OG) in 50 mM phosphate buffer (pH 7.0) with the presence of 10 mM sodium ascorbate. The B820 heterodimers were further purified by means of sucrose density gradient (0.6 -0.8 M) ultracentrifugation (165 000g  16 hrs.…”

Controlling Structural and Functional Features of Photosynthetic Antenna

“…After centrifugation, the B820 subunits were puried on a DEAE--sepharose (Fast Flow, Pharmacia), pre-equilibrated in 1% β-OG (10 mM NaCl/20 mM TRIS·HCl buer, pH 7.8), as described previously [27].…”

“…Rhodospirillum rubrum may constitute up to 32 pigment molecules but smaller assemblies with similar spectral properties can be formed as well [22]. The B780, B820, and B870 forms interact with Crts [23,26,27], which increases the driving force for the assembly (see below). These interactions must be mainly of hydrophobic character because carotenes, such as neurosporene and lycopene, also induce the oligomerization [23,28].…”

“…Crts seem to enhance excitonic coupling between BChla molecules in LH1, as indicated in the shift of Q Y to 880 nm induced by the binding of Crts [6,27].…”