Evidence for efficient uptake and incorporation of sialic acid by eukaryotic cells.

Sialic acids are the most abundant terminal carbohydrate moiety on cell surface glycoconjugates in eukaryotic cells and are of functional importance for many biological ligand-receptor interactions. It is a widely accepted view that sialic acids cannot be efficiently taken up from the extracellular space by eukaryotic cells. To test this assumption, we cultivated two recently identified human hematopoetic cell lines which are hyposialylated due to a deficiency in de novo sialic acid biosynthesis in the presence of N-acetylneuraminic acid (NeuAc), the most frequently found sialic acid. Surprisingly, NeuAc medium supplementation rapidly and potently compensated for the endogenous hyposialylation in a concentration-dependent manner, resulting in the presentation of cell surface sialoglycans involved in cell adhesion, virus infection and signal transduction. We provide several lines of experimental evidence that all suggest that NeuAc was neither extracellularly incorporated nor degraded to a less complex sugar before uptake. Importantly, NeuAc induced a marked increase in intracellular CMP-NeuAc levels in both human cell lines and in primary cells regardless of the prior sialylation status of the cells. Studies employing 9-[3H]NeuAc revealed an uptake consistent with the observed incorporation of unlabeled NeuAc. We propose the existence of an efficient uptake mechanism for NeuAc in eukaryotic cells.

Human papillomavirus type 16 E5 protein localizes to the Golgi apparatus but does not grossly affect cellular glycosylation.

The E5 protein of papillomaviruses is a strongly hydrophobic membrane protein that can associate with the 16 kDa protein subunit of the vacuolar proton ATPase in endosomes and the Golgi apparatus resulting in raise of intraorganelle pH. We demonstrate that E5 of human papillomavirus type 16 (HPV16) when transfected into human keratinocytes localizes to the Golgi. Using FACS analysis and western blotting with a variety of lectins as well as analysing the sialylation status of a specific cell surface glycoprotein CD95 (APO-1/Fas), we show that HPV16 E5 does not grossly affect cellular glycosylation, a main Golgi function.