β-Glucan synthesis in the cotton fiber: III. Identification of UDP-glucose-binding subunits of β-glucan synthases by photoaffinity labeling with [β-³²P]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 (Cossypium hirsutum) cellulose synthase. This polypeptide is enriched by entrapment under conditions favoring β-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 β-1,3-glucan synthase because this polypeptide is the most abundant protein in the entrapment fraction obtained under conditions favoring β-1,3-glucan synthesis, is coincident with β-1,3-glucan synthase activity, and is calcium dependent. The possible involvement of other polypeptides in the synthesis of β-1,3-glucan is discussed.