POLARIZED EXOCYTOSIS IN MDCK CELLS IS REGULATED BY PHOSPHORYLATION

Protein phosphorylation and dephosphorylation systems modulate many ce llular activities and have recently been implicated in the in vitro tr ansport of newly synthesized proteins. Here we show that polarized tra nsport from the Golgi to the plasma membrane in intact MDCK cells is r egulated by phosphorylation-dephosphorylation, Transport is inhibited by the phosphatase inhibitor okadaic acid and is stimulated by the kin ase inhibitor staurosporine. Stimulation of apical transport exceeds s timulation of basolateral transport by up to 5-fold. We also find that the G protein activator aluminum fluoride, which stimulates transport to the surface at low fluoride concentrations as previously reported, inhibits transport at higher concentrations. In the nonpolarized fibr oblast cell line CV-1, neither staurosporine nor aluminum fluoride sti mulates transport to the cell surface. Our results suggest that the ph osphorylation-dephosphorylation system. Like the G protein, may be inv olved in the specialized sorting process characteristic of polarized c ells, We show some evidence that these two mechanisms of regulation ma y act through common intermediates.