Purification of 1,3-beta-D-glucan synthase activity from pea tissue. Two polypeptides of 55 kDa and 70 kDa copurify...

From pea plasma membranes isolated by aqueous polymer two-phase partitioning we have purified 1,3-beta-D-glucan synthase [glucan synthase-II (GS-II) or callose synthase], an enzyme that several reports have suggested consists of between six and nine different subunits. The procedure involves (a) preliminary removal of peripheral proteins by 0.1% digitonin; (b) solubilization of GS-II with 0.5% digitonin; (c) precipitation of activity-irrelevant proteins from the digitonin extract by Ca2+, spermine and cellobiose, which are GS-II effectors needed in step (d); (d) product entrapment by formation of 1,3-beta-D-glucan from UDP-Glc by GS-II in the presence of the mentioned effectors, followed by centrifugal sedimentation of product micelles and elution of proteins therefrom with buffer; (e) preparative isoelectric focusing (IEF) of product-entrapped proteins; and (f) glycerol gradient centrifugation of the fractions of peak GS-II activity from IEF. The procedure yields 300-fold enrichment of GS-II specific activity over that in isolated plasma membranes, and 5500-fold over that in the original homogenate. Out of approximately six principal polypeptides that occur after the product entrapment step, the glycerol gradient GS-II activity peak contains only two major polypeptides, one of 55 kDa and another of 70 kDa, plus minor amounts of one or two others whose distribution and occurrence indicate are not responsible for GS-II activity. Antisera against either the 55-kDa or the 70-kDa polypeptide adsorb more than 60% of the GS-II activity from a product-entrapped preparation. After native gel electrophoresis, GS-II activity is associated with a single protein band of very large molecular mass, whose principal components are the 55-kDa and 70-kDa polypeptides, accompanied by minor amounts of a few other polypeptides most of which do not occur in enzyme preparations purified by the previously described procedure. The 55-kDa but not the 70-kDa component can be labeled by ultraviolet irradiation of the plasma membranes in the presence of [alpha-32P]UDP-Glc under GS-II assay conditions. It seems likely, therefore, that the 55-kDa and 70-kDa polypeptides form a large catalytic complex of which the 55-kDa component is the UDP-Glc-binding subunit.