Detergent-solubilized microsomal proteins from flax can use an RG-I-enriched fraction as an exogenous acceptor for methylation in the presence of S-adenosylmethione (334). The pectin methyltransferase activity was stimulated by the addition of the enriched RG-I fraction 1.5- to 1.7-fold above levels recovered using endogenous acceptor, and the radio­labeled product had a size similar to RG-I. However, since it was not shown where in RG-I the methylation occurred, it is not clear whether the methylation occurred on GalA in the RG-I backbone or, rather, on possible HG tails that may have been covalently linked to RG-I. Also, it was not established whether some of the methylation may have occurred on a non-galacturonic substituent such as methylation at the 4-position of glucuronic acid in the side branched of RG-I (278).

5.4.10.2 RG-I:acetyltransferase (RG-I:AT)

GalA in the alternating [^4)-a-D-GalpA-(1^2)- a-L-Rhap-(1^] backbone of RG-I may be acetylated on C-2 and/or C-3 (289). Microsomal membranes from suspension — cultured potato cells (339) contain an RG-I acetyltransferase that transfers [14C]acetate from [14C]acetyl-CoA onto endogenous RG-I acceptor to yield a >500-kDa radiolabeled product (339), based on the release of [14C]acetate following incubation of the product with a purified rhamnogalacturonan O-acetyl esterase, and fragmentation of the product by rhamnogalacturonan lyase (RGase B) (339). The RG-I acetyltransferase has an apparent Km for acetyl-CoA of 35 ^M and a pH optimum of 7.0.