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Metabolism and Enzymology

Direct Photolabeling with [³²P]UDP-Glucose for Identification of a Subunit of Cotton Fiber Callose Synthase ¹

Department of Botany, Institute of Life Sciences, The Hebrew University of Jerusalem, 91904, Israel

We have identified a 52 kilodalton polypeptide as being a likely candidate for the catalytic subunit of the UDP-glucose: (13)--glucan (callose) synthase of developing fibers of Gossypium hirsutum (cotton). Such a polypeptide migrates coincident with callose synthase during glycerol gradient centrifugation in the presence of EDTA, and can be directly photolabeled with the radioactive substrate, -[³²P]UDP-glucose. Interaction with the labeled probe requires Ca²⁺, a specific activator of callose synthase which is known to lower the K_m of higher plant callose synthases for the substrate UDP-glucose. Using this probe and several other related ones, several other proteins which interact with UDP-glucose were also identified, but none satisfied all of the above criteria for being components of the callose synthase.

¹ This work was supported by contract DE-ACO2-76ERO-1338 from The U.S. Department of Energy via a subcontract from Michigan State University, East Lansing, MI, and by a grant from the German-Israeli Foundation for Scientific Research and Development (GIF).

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