Effects of substrate and potassium on the betaine-synthesizing enzyme glycine sarcosine dimethylglycine N-methyltransferase from a halophilic methanoarchaeon Methanohalophilus portucalensis

Methanohalophilus portucalensis FDF1 can synthesize the compatible solute betaine de novo through the methylation of glycine, sarcosine and dimethylglycine with the methyl group from S-adenosylmethionine. After separation by DEAE-Sephacel ion chromatography using a KCl step gradient, glycine, sarcosine and dimethylglycine methytransfer (GSDMT) activities were detected in a single peak. The estimated molecular weight of GSDMT was 240 kDa and 2-D gel analysis indicated it was separated into four subunits (52 kDa) with different pI. The PBE94 chromatofocusing column also separated GSDMT into four protein peaks A, B, C, D. Both peak B and D proteins possessed GSDMT activity, while the peak A protein only exhibited SDMT activity. The multiple methyltransferase activities of the large complex appear to be unique compared to other methyltransferases used in betaine synthesis. Further methyltransferase assays in response to different concentrations of KCl indicated that the peak D protein exhibited low GSDMT activity only when K+⩽0.4 M. The peak B protein exhibited a higher GSDMT activity at 0.4 M K+, while the peak A protein exhibited SDMT activity only at higher K+ (0.8 M). These results suggest that the internal K+ concentration regulates GSDMT activities and affects the net betaine accumulation in the cells.