Polymerization of the backbone of the pectic polysaccharide rhamnogalacturonan I

Abstract

Rhamnogalacturonan I (RG-I) is a major plant cell wall pectic polysaccharide defined by its repeating disaccharide backbone structure of [4)-α-d-GalA-(1,2)-α-l-Rha-(1,]. A family of RG-I:Rhamnosyltransferases (RRT) has previously been identified, but synthesis of the RG-I backbone has not been demonstrated in vitro because the identity of Rhamnogalacturonan I:Galaturonosyltransferase (RG-I:GalAT) was unknown. Here, a putative glycosyltransferase, At1g28240/MUCI70, is shown to be an RG-I:GalAT. The name RGGAT1 is proposed to reflect the catalytic activity of this enzyme. When incubated together with the rhamnosyltransferase RRT4, the combined activities of RGGAT1 and RRT4 result in elongation of RG-I acceptors in vitro into polymeric product. RGGAT1 is a member of a new GT family categorized as GTx that is phylogenetically distinct from the GALACTURONOSYLTRANSFERASE (GAUT) family of GalA transferases that synthesize the backbone of the pectin homogalacturonan (HG). RGGAT1 has a predicted GT-A fold structure, but employs a metal-independent catalytic mechanism that is rare among glycosyltransferases with this fold type, suggesting that RG-I biosynthesis has functionally diverged from the mechanisms used for synthesis of HG and other plant cell wall polysaccharides.