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Heterologous expression and initial characterization of recombinant RbcX protein from Thermosynechococcus elongatus BP-1 and the role of RbcX in RuBisCO assembly

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Tarnawski M., Gubernator B., Kolesinski P., Szczepaniak A.
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2008
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vol. 55
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issue 4
777-785
EN
In the cyanobacterial RuBisCO operon from Thermosynechococcus elongatus the rbcX gene is juxtaposed and cotranscribed with the rbcL and rbcS genes which encode large and small RuBisCO subunits, respectively. It has been suggested that the rbcX position is not random and that the RbcX protein could be a chaperone for RuBisCO. In this study, the RbcX protein from T. elongatus was overexpressed, purified and preliminary functional studies were conducted. The recombinant protein purified from Escherichia coli extracts was predominantly present in a soluble fraction in a dimeric form. Coexpression experiments have demonstrated that RbcX can mediate RbcL dimer (L2) formation, and that it is essential for the L8 core complex assembly. This is the first characterization of the RbcX protein from a thermophilic organism.
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On the mode of integration of the thylakoid membrane protein cytochrome b6 into cytoplasmic membrane of Escherichia coli

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Króliczewski J., Gubernator B., Rögner M., Szczepaniak A.
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2011
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vol. 58
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issue 3
335-343
EN
In the stroma compartment, several pathways are used for integration/translocation of chloroplast proteins into or across the thylakoid membrane. In this study we investigated the mode of incorporation of the chloroplast-encoded cytochrome b6 into the bacterial membrane. Cytochrome b6 naturally comprises of four transmembrane helices (A,B,C,D) and contains two b-type hemes. In the present study, mature cytochrome b6 or constructed deletion mutants of cytochrome were expressed in E. coli cells. The membrane insertion of cytochrome b6 in this bacterial model system requires an artificially added presequence that directs the protein to use an E. coli membrane-insertion pathway. This could be accomplished by fusion to maltose-binding protein (MBP) or to the bacterial Sec-dependent signal peptide (SSpelB). The integration of mature cytochrome b6 into the bacterial cytoplasmic membrane by the Sec pathway has been reported previously by our group (Kroliczewski et al., 2005, Biochemistry, 44: 7570). The results presented here show that cytochrome b6 devoid of the first helix A can be inserted into the membrane, as can the entire ABCD. On the other hand, the construct devoid of helices A and B is translocated through the membrane into the periplasm without any effective insertion. This suggests the importance of the membrane-anchoring sequences that are likely to be present in only the A and B part, and it is consistent with the results of computational prediction which did not identify any membrane-anchoring sequences for the C or D helices. We also show that the incorporation of hemes into the truncated form of cytochrome b6 is possible, as long as the B and D helices bearing axial ligands to heme are present.
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