Apical secretion of a cytosolic protein by Madin-Darby canine kidney cells. Evidence for polarized release of an endogenous lectin by a nonclassical secretory pathway.

In the classical secretory pathway proteins containing a signal peptide are translocated from the cytoplasm of the cell into the lumen of the endoplasmic reticulum (ER). From the ER they are transported to the Golgi apparatus and finally to the plasma membrane (PM) where they are released into the extracellular compartment. However, some proteins are synthesized without a signal peptide and maintain a predominantly cytosolic distribution until they are released from the cell. As a marker for this nonclassical secretory pathway we have chosen L-29, a soluble lectin of M(r) about 29,000, that has affinity for lactose and other beta-galactoside containing glycoconjugates. We were interested in determining if cultured epithelial cells secrete L-29 and if they do so in a polarized fashion. Madin-Darby canine kidney (MDCK)-II cells were found to express large quantities of L-29 (about 1% of the detergent soluble protein). The lectin was diffusely distributed in the cytosol, with little or none in vesicular compartments. The polarity of L-29 secretion, when analyzed in pulse-chase experiments, was selectively into the apical compartment of filter-grown MDCK cells. This secretion was not inhibited by brefeldin A or monensin, drugs that are known to inhibit protein transport through the ER-Golgi-PM pathway. Secretion of L-29 was augmented 3-5-fold by the calcium ionophore A23187 and by increasing the temperature to 42 degrees C, whereas lowering the temperature to 20 degrees C or addition of nocodazole prevented secretion. These results demonstrate the polarized secretion of a cytosolic protein by a nonclassical secretory pathway.