Intracellular degradation of glycoproteins in Acer pseudoplatanus L. cells

Abstract

The degradation of endogenously labelled glycoproteins was studied in Acer pseudoplatanus L. cell suspension cultures in experiments using a dual-label with [ 14C]mannose and [ 3H]leucine. After harvesting the cells, protoplasts were prepared and vacuoles isolated. More than 30% of both total newly synthesized proteins ( 3H radioactivity) and glycoproteins ( 14C radioactivity) were recovered inside the vacuoles, the lytic compartment of plant cells. Half of these proteins were degraded when isolated vacuoles were incubated for 6 h at 20°C. So, the vacuolar compartment appears to be a major site of glycoprotein degradation in the cell. The glycoproteins were degraded at the same rate as the total newly synthesized proteins. However, some vacuolar hydrolytic enzymes were found to be glycoproteins and resistant to proteolytic attack. The biochemical explanation for such a resistance is not clear at this time, but in Acer cells the presence of covalently bound carbohydrates in proteins does not seem to be involved in the selectivity of protein turnover.