N-glycans are not a universal signal for apical sorting of secretory proteins

In MDCK cells, N-glycans have been shown to determine the sorting of secret ory proteins and membrane proteins to the apical domain in the absence of a dominant basolateral targeting signal. We have examined the sorting of end ogenous proteins in ECV304 cells in the presence and absence of tunicamycin , an inhibitor of N-linked glycosylation, A prominent apically secreted pro tein of 71 kDa was not N-glycosylated and continued to be secreted apically in the presence of tunicamycin, In contrast, other endogenous proteins tha t were N-glycosylated mere secreted preferentially into the basolateral med ium or without polarity, When rat growth hormone was expressed in MDCK and ECV304 cells, we observed 65 and 94% of the secretion to the basolateral me dium, respectively, Introduction of a single N-glycan caused 83% of the gro wth hormone to be secreted at the apical surface in MDCK cells but had no s ignificant effect on the polarity of secretion of growth hormone in ECV304 cells, These results indicate that not all cell lines recognise N-glycans a s a signal for apical sorting and raises the possibility of using ECV304 ce lls as a model system for analysis of apical sorting molecules. (C) 1999 Fe deration of European Biochemical Societies.