Catalytic mechanisms

Although there is considerable evidence that the GSL proteins are involved in (1,3)-P-d-glucan synthesis and that they can use UDP-Glc as a source of glucosyl residues, they do not contain a recognizable UDP-Glc binding motif. There has been some debate as to whether the GSL proteins constitute the catalytic subunit itself, or whether they are simply a pore-forming unit in a larger (1,3)-P-d-glucan synthase complex. The GSL proteins have some structural similarity to bacterial and eukaryotic transporters (Douglas et al. 1994 ; Cui et al. 2001b ; Hong et al. 2001a ; Dijkgraaf et al. 2002 ). However, it remains possible that GSL proteins are catalytically active but use a novel motif for UDP-Glc binding. The (1,3)-P-d-glucan synthases of plants may consist of multiprotein complexes. Thus, the barley GSL1 protein has been identified in high- molecular- mass complexes on gels (Li et al. 2003 ), but other polypeptides were also present in the region containing the GSL proteins. Potential complex components have been identified by their interaction with GSL proteins. The GhGSL1 protein from cotton fibres binds calmodulin in vitro in the presence of Ca2+ (Cui et al. 2001b) and yeast-two-hybrid analyses indicate that AtGSL6, the putative cell-plate callose synthase, interacts with phragmoplastin (a cell-plate-associated dynamin-like protein) and a novel UDP-Glc transferase (Hong et al. 2001b). Furthermore, (1,3)-P-d-glucans can form triple helices of parallel chains (Stone & Clarke 1992; Pelosi et al. 2003) and although these might form spontaneously, it is formally possible that they could result from the activity of associated catalytic subunits (Lai-Kai-Him et al. 2001).

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