Abstract

Cytochrome c maturation (Ccm) is a post-translational and post-export protein modification process that involves ten (CcmABCDEFGHI and CcdA or DsbD) components in most Gram-negative bacteria. The absence of any of these components abolishes the ability of cells to form cytochrome c, leading in the case of Rhodobacter capsulatus to the loss of photosynthetic proficiency and respiratory cytochrome oxidase activity. Based on earlier molecular genetic studies, we inferred that R. capsulatus CcmF, CcmH, and CcmI interact with each other to perform heme-apocytochrome c ligation. Here, using functional epitope-tagged derivatives of these components coproduced in appropriate mutant strains, we determined protein-protein interactions between them in detergent-dispersed membranes. Reciprocal affinity purification as well as tandem size exclusion and affinity chromatography analyses provided the first biochemical evidence that CcmF, CcmH, and CcmI associate stably with each other, indicating that these Ccm components form a membrane-integral complex. Under the conditions used, the CcmFHI complex does not contain CcmG, suggesting that the latter thio-reduction component is not always associated with the heme ligation components. The findings are discussed with respect to defining the obligatory components of a minimalistic heme-apocytochrome c ligation complex in R. capsulatus.

Highlights

  • Mitochondria of plants and protozoa (Ccm-system I) (2)

  • In most Gram-negative bacteria, Cytochrome c maturation (Ccm)-system I consists of ten components, CcmABCDEFGHI and CcdA, acting on the outer side of the cytoplasmic membrane (3), and ligating heme molecules to apocyts c following their cytoplasmic syntheses

  • In this work, using combinations of reciprocal affinity and size exclusion chromatography, we provide the first direct biochemical evidence that R. capsulatus CcmF, CcmH, and CcmI interact with each other to form a stable, multisubunit membrane protein complex

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Summary

EXPERIMENTAL PROCEDURES

Plasmid pCS1718 was digested with NdeI and BamHI sites, which were introduced during the PCR amplification from pHM14, and the DNA fragment corresponding to cycAmat cloned into the same sites of pCS1302, a derivative of pCS905 (28), to yield pCS1726. This plasmid contains an inframe Strep-tag sequence fused at the 5Ј-end of cycAmat expressed from a Ptac-lac promoter-operator system in E. coli. For tandem size exclusion and affinity chromatography, high molecular weight fractions separated by the size exclusion column were pooled, the TNED1 buffer was exchanged with TNED2, and proteins were concentrated to 1 mg/ml protein using 10-kDa cut-off Centriplus YM-10 centrifugal filter units (Millipore, Billerica, MA).

Relevant phenotype
Pico chemiluminescent substrate
RESULTS
DISCUSSION
Full Text
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