RESUMO
PufX, the protein encoded by the pufX gene of Rhodobacter capsulatus and Rhodobacter sphaeroides, has been further characterized. The mature forms of these proteins contain 9 and 12 fewer amino acids, respectively, at the C-terminal end of the protein than are encoded by their pufX genes. To identify the portion of PufX responsible for inhibition of LH1 formation in reconstitution experiments, different regions (N-terminus and several core regions containing different lengths of the C-terminus) of Rb. sphaeroides and Rb. capsulatus PufX were chemically synthesized. Neither the N- nor C-terminal polypeptides of Rb. sphaeroides were inhibitory to LH1 reconstitution. However, all core segments were active, causing 50% inhibition at a concentration ratio of between 3:1 and 6:1 relative to the LH1 alpha-polypeptides whose concentrations were 3-4 microM. CD measurements indicated that the core segment containing 39 amino acids of Rb. sphaeroides PufX exhibited 47% alpha-helix in trifluoroethanol while the core segment containing 43 amino acids of Rb. capsulatus PufX exhibited 59 and 55% alpha-helix in trifluoroethanol and in 0.80% octylglucoside in water, respectively. Approximately 50% alpha-helix was also indicated by a PHD (Burkhard-Rost) structure prediction. Binding of bacteriochlorophyll to these PufX core segments is implicated.
Assuntos
Proteínas de Bactérias/química , Bacterioclorofilas/antagonistas & inibidores , Complexos de Proteínas Captadores de Luz , Complexo de Proteínas do Centro de Reação Fotossintética/antagonistas & inibidores , Rhodobacter capsulatus/metabolismo , Rhodobacter sphaeroides/metabolismo , Sequência de Aminoácidos , Proteínas de Bactérias/síntese química , Proteínas de Bactérias/isolamento & purificação , Proteínas de Bactérias/metabolismo , Bacterioclorofilas/metabolismo , Cromatografia Líquida de Alta Pressão , Dicroísmo Circular , Modelos Moleculares , Dados de Sequência Molecular , Fragmentos de Peptídeos/síntese química , Fragmentos de Peptídeos/isolamento & purificação , Fragmentos de Peptídeos/metabolismo , Complexo de Proteínas do Centro de Reação Fotossintética/metabolismo , Estrutura Secundária de Proteína , Estrutura Terciária de Proteína , Rhodobacter capsulatus/química , Rhodobacter sphaeroides/química , Especificidade da EspécieRESUMO
Light and dark modulation experiments with pea (Pisum sativum L.) chloroplast stromal fractions pretreated with dithiothreitol (to reduce protein disulfide bonds) or with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) (to block sulfhydryl groups) suggest that light modulation involves thiol-disulfide exchange on the modulatable stromal enzyme protein. Light-dependent reduction of DTNB involves a photosynthetic electron transport chain component located on the reducing side of photosystem I prior to ferredoxin; DTNB may be acting as a light effect mediator substitute. The thylakoid-bound light effect mediator system, then, in its light-activated reduced form probably catalyzes thiol-disulfide exchange reactions on stromal enzymes.