Competition of Xpt1p activity. Xylosyltransferase activity assays were performed using detergent extracts of KN99α cxt1Δ cxt2Δ cells (see “Experimental Procedures”). All reactions contained standard amounts of UDP-[14C]Xyl, α-1,3-Man2, and MnCl2 with varying amounts of UDP-Xyl (A) or GDP-Man (B). Lane 1, 0 μm; lane 2, 1 μm; lane 3, 5 μm; lane 4, 10 μm; lane 5, 25 μm; lane 6, 50 μm; lane 7, 75 μm; lane 8, 100 μm; lane 9, 250 μm; lane 10, 500 μm. Only the region of the TLC plate with the Xpt1p product is shown; no signal was detected in other regions. F, solvent front.