BETA-GLUCAN SYNTHESIS IN THE COTTON FIBER .3. IDENTIFICATION OF UDP-GLUCOSE-BINDING SUBUNITS OF BETA-GLUCAN SYNTHASES BY PHOTOAFFINITY-LABELING WITH [BETA-P-32]5'-N3-UDP-GLUCOSE

Using differential product entrapment and photolabeling under specifying conditions, we identified a 37-kD polypeptide as the best candidate among the UDP-glucose-binding polypeptides for the catalytic subunit of cotton (Gossypium hirsutum) cellulose synthase. This polypeptide is enriched by entrapment under conditions favoring beta-1,4-glucan synthesis, and it is magnesium dependent and sensitive to unlabeled UDP-glucose. A 52-kD polypeptide was identified as the most likely candidate for the catalytic subunit of beta-1,3-glucan synthase because this polypeptide is the most abundant protein in the entrapment fraction obtained under conditions favoring beta-1,3-glucan synthesis, is coincident with beta-1,3-glucan synthase activity, and is calcium dependent. The possible involvement of other polypeptides in the synthesis of beta-1,3-glucan is discussed.