Synthesis of membrane glycoproteins in rat small-intestinal villus cells. Redistribution of l-[1,5,6-3H]-fucose-labelled membrane glycoproteins among Golgi, lateral basal and microvillus membranes in vivo

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Abstract
The biogenesis of plasmalemma glycoproteins of rat small-intestinal villus cells was studied by following the incorporation of L-[1,5,6-3H]fucose, given i.p. with and without chase, into Golgi, lateral basal and microvillus membranes. Each membrane fraction showed distinct kinetics of incorporation of labeled fucose and was differently affected by the chase, which produced a much greater decrease in incorporation of label into Golgi and microvillus than into lateral basal membranes. The kinetic data suggest a redistribution of newly synthesized glycoproteins from the site of fucosylation, the Golgi complex, directly into both lateral basal and microvillus membranes. The observed biphasic pattern of label incorporation into the microvillus membrane fraction may be evidence for a 2nd indirect route of incorporation. The selective effect of the chase suggests the presence of 2 different pools of radioactive fucose in the Golgi complex that differ in their accessibility to dilution with non-radioactive fucose, and their utilization for the biosynthesis of membrane glycoproteins subsequently destined for either the microvillus or the lateral basal parts of the plasmalemma. The radioactively labeled glycoproteins of the different membrane fractions were separated by sodium dodecyl sulfate/polyacrylamide-slab-gel electrophoresis and identified by fluorography. The patterns of labeled glycoproteins in Golgi and lateral basal membranes were identical at all times. At least 14 bands could be identified shortly after radioactive-fucose injection. Most seemed to disappear at later times, although one of them, which was never observed in microvillus membranes, increased in relative intensity. All but 2 of the labeled glycoproteins present in the microvillus membrane corresponded to those observed in Golgi and lateral basal membranes shortly after fucose injection. The patterns of labeled glycoproteins in all membrane fractions were little affected by the chase. These data support a flow concept for the insertion of most surface-membrane glycoproteins of the intestinal villus cells.