Structural and spectral characterisation of the antenna complexes of Rhodocyclus gelatinosus

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Abstract
The core antenna/reaction‐centre complex RC‐B875 and the peripheral antenna complex B800–850 of the two strains DSM 149 and DSM 151 of the purple non‐sulphur bacterium Rhodocyclus gelatinosus have been isolated from photosynthetic membranes by means of lauryl‐N,N‐dimethyl‐amineoxide as a detergent and subsequent sucrose‐gradient centrifugation. The two complexes were characterised spectroscopically by absorption and circular dichroism (CD) spectroscopy at room temperature. CD measurements revealed very weak signals for the core antenna B875 whereas for the peripheral antenna B800–850, a strong biphasic CD signal was observed, attributable to the B850 pigments. There is apparently no CD signal present for the B800 pigments. The core and the peripheral antenna complex are built up by a distinct α/β‐polypeptide pair. The pigment/protein ratio in the peripheral antenna complex is 3 bacteriochlorophyll/(α/β)‐polypeptide pair. The amino acid sequences of the α and β polypeptides of both complexes from the two strains of Rc. gelatinosus were established by automated Edman degradation, chemical and enzymic digestion, amino acid composition analyses and carboxypeptidase digestion. In the case of the β polypeptides, the amino acid sequence determination was confirmed by ion‐spray MS of the isolated antenna apoproteins. The inter‐strain (DSM 149 and 151) positional identity between the equivalent apoproteins is extremely large and varies in the range 90–100%. The B875‐β polypeptide from Rc. gelatinosus exhibits shortened C‐termini, as detected for the analogous antenna apoproteins of Rhodobacter sphaeroides and Rhodobacter capsulatus, which can be correlated with weak core antenna near‐infrared CD signals. However, the B800–850‐α polypeptide of Rc. gelatinosus, with 71 amino acids, exhibits an extended C‐terminal portion indicative of the formation of a second transmembrane domain, which so far has not been observed for bacterial antenna apoproteins. This part of the molecule is extremely rich in alanine and proline residues. All the sequenced antenna apoproteins of Rc. gelatinosus exhibit a characteristic membrane‐buried histidine which is thought to ligate the B875 or the B850 pigments. In the B800–850‐β apoprotein, a second, so far β‐antenna‐apoprotein‐specific histidine, is replaced by a glutamine residue. A careful inspection of the determined antenna structures of Rc. gelatinosus revealed some remarkable structural similarities within presumed cofactor‐binding sites of Fe‐S‐type‐reaction‐centre apoproteins, indicating possible basic structural motifs for complexing bacteriochlorophyll molecules.

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