The periplasmic modifier protein for methanol dehydrogenase in the methylotrophs Methylophilus methylotrophus and Paracoccus denitrificans.

A modifier protein (M-protein), which increases the affinity of methanol dehydrogenase (MDH) for alcohols but decreases its affinity for formaldehyde, has been partially purified from Methylophilus methylotrophus and Paracoccus denitrificans. Analysis was complicated by non-protein factors in bacterial extracts that are able to mimic M-protein in one of its functions-that of increasing the activity of MDH with butane-1,3-diol in the dye-linked assay system. The 67 kDa polypeptide, previously identified as a subunit of the M-protein, is an unrelated cytoplasmic protein. The M-protein is exclusively periplasmic and is a multimeric protein with subunits of 45 kDa. The M-protein is active in the 'physiological' assay system with the specific cytochrome c electron acceptor for MDH, lowering its affinity for formaldehyde. It has its maximum effect when the ratio of M-protein:MDH is 1:5 but its concentration in the periplasm is much lower than 20% of that of MDH.