HG-methyltransferase (HG-MT) catalyzes the methylesterification of HG at the C-6 car­boxyl group of some GalA residues by transfer of a methyl group from S-adenosylmethionine to HG. The name HG-MT is preferred, rather than the former name pectin methyltransferase, to distinguish HG-MT from the enzymes that methylate RG-I or RG — II. HG-MT activity has been identified in microsomal membranes from mung bean (280, 281, 283), flax (227, 282), tobacco (284), and soybean (285). Membrane-bound HG-MTs from flax (333, 334), and tobacco (335) could be solubilized using detergent. HG-MT has been localized to the Golgi (227-229, 336) and its catalytic site has been shown to face the Golgi lumen (229). In vitro biochemical studies showing that UDP-GalA stimulates HG-MT activity in intact membrane vesicles (284, 308) and that polygalacturonic and pectin can function in vitro as acceptors for HG-MT in detergent-permeabilized membranes support a model in which at least a small stretch of HG is synthesized prior to its methylation by HG-MT in the Golgi. The observation that some of the HG-MTs in detergent-permeabilized membranes from flax and soybean show a preference for partially esterified pectin (228,285, 337) over polygalacturonic acid further suggest that multiple HG-MTs may exist that differ in their specificity for HG of differing degrees of methylation. The question of whether such HG-MTs are preferentially involved in the initial methylation of HG or in the methylation of more highly esterified HG remains to be resolved. The observation that overexpression of an Arabidopsis S-adenosylmethionine (SAM) synthetase gene in flax leads to a concomitant increase in SAM synthetase activity and of pectin methylesterification in the wall, with no increase in HG-MT activity, suggests that the degree of methylesterification of HG maybe regulated, at least in part, at the level of substrate (i. e., SAM) concentration (338).

The gene encoding HG-MT has not yet been unambiguously identified. Two apparent HG-MT isozymes, PMT5 and PMT7, were reported from flax (337) and efforts to purify these apparent isozymes resulted in the identification of an additional small polypeptide with HG-MT activity designated PMT18. The definitive identification of one or more of these polypeptides as HG-MT has not yet been reported. Thus, the proposition that the 18-kDa protein is a subunit of the 40- and 110-kDa proteins (337) has not been substantiated.

Recently, Mouille and colleagues (287) have identified a putative methyltransferase as the gene mutated in the Arabidopsis mutant quasimodo2. Qua2-1 plants are dwarfed and have a 50% reduction in HG. Although confirmation of enzyme activity of QUA2 is required to establish if it indeed encodes an HG-MT, the reduced HG phenotype of qua2 plants along with the Golgi localization of QUA2-GFP fusions and the putative methyltransferase domain in QUA2 are consistent with a role as an HG-MT. Further work on QUA2 and the 29 QUA2-related proteins in Arabidopsis may shed light on the identity of multiple methyltransferases required for pectin synthesis.