Functional characterization of carrier‐mediated transport of uridine diphosphate N‐acetylglucosamine across the endoplasmic reticulum membrane

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Abstract
Uptake and metabolism of uridine diphosphate N‐acetylglucosamine (UDP‐GlcNAc) by rough‐endoplasmic‐reticulum (rER)‐derived vesicles was studied. Analysis of the molecular species, double‐label experiments, cis‐inhibition and trans‐stimulation experiments revealed that uptake represented entry of intact UDP‐GlcNAc into the microsomal lumen. The amount of vesicle‐associated label at equilibrium uptake was directly proportional to the volume of the intravesicular space, and permeabilized microsomes were unable to retain UDP‐GlcNAc. These findings indicate that uptake constituted effective translocation from the medium into the lumen of the vesicles. The microsomal uptake of UDP‐GlcNAc met the criteria of bidirectional, carrier‐mediated translocation. Transport was time and temperature dependent, saturable, selective, capable of trans‐stimulation and operational against a concentration gradient. Uptake studies performed in membrane preparations that were highly enriched in either rER, smooth ER, or Golgi revealed that UDP‐GlcNAc was taken up by the ER and by the Golgi apparatus.