Polypeptide assemblies cross-linked by S-S bonds (molecular mass 200 kDa) and single polypeptides folded with internal S-S cross-links ( 41 kDa) have been detected by SDS\PAGE in particulate membranes and soluble extracts of developing cotyledons of nasturtium (Tropaeolum majus L.). When first prepared from fruit homogenates, these polypeptides were found to bind reversibly to UDP-Gal (labelled with ["%C]Gal or [$H]uridine), and to co-precipitate specifically with added xyloglucan from solutions made with 67 % ethanol. Initially, the bound UDP-["%C]Gal could be replaced (bumped) by adding excess UDP, or exchanged (chased) with UDP-Gal, -Glc, -Man or -Xyl. However, this capacity for turnover was lost during incubation in reaction media, or during SDS\PAGE under reducing conditions, even as the glycone moiety was conserved by autoglycosylation to form a stable 41 kDa polypeptide.