TOPOGRAPHY AND FUNCTION OF GOLGI URIDINE-5'-DIPHOSPHATASE FROM PEA STEMS

Golgi UDPase is an enzyme that has been shown to function in polysacch aride biosynthesis, but its role in this process is not yet clear. In this study rye identified Golgi UDPase activity in pea (Pisum sativum) stems and differentiated it from another UDPase activity. We demonstr ated that Golgi UDPase is an integral membrane protein, based on speci fic partitioning of this activity into Triton X-114. Analysis of its t opology using sealed, right-side-out Golgi vesicles and treatment with proteinase K suggested that its active site faces the Golgi lumen. St udies aimed at understanding the function of Golgi UDPase by incubatin g Golgi vesicles with [beta-P-32]UDP-glucose (Glc) to generate [beta-P -32]UDP upon Glc transfer in situ showed that P-32(i), but not [beta-P -32]UDP, was formed, suggesting that UDPase quickly hydrolyzed the UDP formed during Clc polymerization. We found that the Colgi UDPase was highly active in the elongating region of the third internode, whereas no activity was detected in the first and second internodes of etiola ted pea seedlings. These results suggest that UDPase removes the UDP f ormed during Glc polymerization and could be important in the mechanis m of polysaccharide biosynthesis.