Pectin biosynthesis: a solubilized alpha 1,4-galacturonosyltransferase from tobacco catalyzes the transfer of galacturonic acid from UDP-galacturonicacid onto the non-reducing end of homogalacturonan

A solubilized al,4-galacturonosyltransferase (GalAT) from tobacco transfers galacturonic acid (GalA) residues from UDP-GalA onto oligogalacturonide (O GA) exogenous accepters with degrees of polymerization greater than nine (R .L. Doong and D. Mohnen 1998, Plant J 13. 363-374). The solubilized GalAT h as been identified as putative polygalacturonate 4-alpha-galacturonosyltran sferase (PGA-GalAT, EC 2.4.1.43) based on its alpha 1,4-galacturonosyltrans ferase activity and similar K-m for UDP-GalA, pH optimum and V-max to those of membrane-bound PGA-GalAT (R.L. Doong et al., 1995, Plant Physiol 109. 1 41-152). The direction of elongation of homogalacturonan catalyzed by solub ilized GalAT from microsomes of tobacco (Nicotiana tabacum L. cv. Samsun ce ll suspensions has now been determined. Three different types of exogenous acceptor were used to study the direction of synthesis of homogalacturonan: unmodified OGAs, OGAs derivatized by biotinylation at the reducing end, an d OGAs containing a 4,5-unsaturated GalA at the non-reducing end. The unmod ified OGAs and the OGAs modified at the reducing end functioned equally wel l as accepters in the galacturonosyltransferase reaction. In contrast, OGAs with the 4,5-unsaturated residue at the non-reducing end were not accepter s for homogalacturonan biosynthesis. These results show that homogalacturon an biosynthesis by solubilized GalAT occurs via the addition of GalA to the non-reducing end of the polymer chain.