BchJ and BchM interact in a 1 : 1 ratio with the magnesium chelatase BchH subunit of Rhodobacter capsulatus.
Substrate channeling between the enzymatic steps in the (bacterio)chlorophyll biosynthetic pathway catalyzed by magnesium chelatase (BchI/ChlI, BchD/ChlD and BchH/ChlH subunits) and S-adenosyl-L-methionine:magnesium-protoporphyrin IX O-methyltransferase (BchM/ChlM) has been suggested. This involves delivery of magnesium-protoporphyrin IX from the BchH/ChlH subunit of magnesium chelatase to BchM/ChlM. Stimulation of BchM/ChlM activity by BchH/ChlH has previously been shown, and physical interaction of the two proteins has been demonstrated. In plants and cyanobacteria, there is an added layer of complexity, as Gun4 serves as a porphyrin (protoporphyrin IX and magnesium-protoporphyrin IX) carrier, but this protein does not exist in anoxygenic photosynthetic bacteria. BchJ may play a similar role to Gun4 in Rhodobacter, as it has no currently assigned function in the established pathway. Purified recombinant Rhodobacter capsulatus BchJ and BchM were found to cause a shift in the equilibrium amount of Mg-protoporphyrin IX formed in a magnesium chelatase assay. Analysis of this shift revealed that it was always in a 1 : 1 ratio with either of these proteins and the BchH subunit of the magnesium chelatase. The establishment of the new equilibrium was faster with BchM than with BchJ in a coupled magnesium chelatase assay. BchJ bound magnesium-protoporphyrin IX or formed a ternary complex with BchH and magnesium-protoporphyrin IX. These results suggest that BchJ may play a role as a general magnesium porphyrin carrier, similar to one of the roles of GUN4 in oxygenic organisms.
Department of Chemistry and Biomolecular Sciences, Macquarie University, Sydney, NSW, Australia.
Artur
A
Sawicki
Robert D
RD
Willows
D002851:Chromatography, High Pressure Liquid / Q000379:methods; D008190:Lyases / Q000235:genetics / Q000378:metabolism*; D008274:Magnesium / Q000737:chemistry / Q000378:metabolism; D008780:Methyltransferases / Q000378:metabolism; D015394:Molecular Structure; D011136:Polysorbates / Q000737:chemistry; D021122:Protein Subunits / Q000235:genetics / Q000378:metabolism*; D011524:Protoporphyrins / Q000737:chemistry / Q000378:metabolism; D016354:Rhodobacter capsulatus / Q000201:enzymology*; D013501:Surface-Active Agents / Q000737:chemistry
D011136:Polysorbates; D021122:Protein Subunits; D011524:Protoporphyrins; D013501:Surface-Active Agents; C028025:protoporphyrin IX; D008780:Methyltransferases; D008190:Lyases; C086788:magnesium chelatase; D008274:Magnesium