Third the 83 kd band is labeled only when it is

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there is already some present in the enzyme preparation. Third, the 83-kd band is labeled only when it is exposed to UV light. By contrast, the 57-kd band is labeled in the presence or absence of UV light. Finally, excess UDP- glucose, the normal substrate, inhibits labeling of the 83-kd band. Labeling of the 57-kd band is not affected by excess UDP-glucose. These results argue convincingly that the 83-kd band is cellulose synthase. Additional studies revealed that the 57-kd band was phosphoglucomu- tase, which catalyzes the interconversion of glucose 1-phosphate and glu- cose 6-phosphate. The enzyme catalyzes this reaction by forming a covalent intermediate with the substrate, which is the basis for non-UV light-depen- dent cross-linking. It is thought that the substrate for this reaction arises from the breakdown of 32 P-azido-UDP-glucose, or as a contaminant in the preparation of the photoaffinity label. Reference: Lin FC, Brown RM, Drake RR & Haley BE (1990) Identification of the uridine 5 ¢ -diphosphoglucose (UDP-Glc) binding subunit of cellulose synthase in Acetobacter xylinum using the photoaffinity probe 5-azido- UDP-Glc. J. Biol. Chem. 265, 4782–4784. THE PLANT CELL WALL A455
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