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J Biol Chem, Vol. 274, Issue 44, 31632-31640, October 29, 1999

Carboxypeptidase M, a Glycosylphosphatidylinositol-anchored Protein, Is Localized on Both the Apical and Basolateral Domains of Polarized Madin-Darby Canine Kidney Cells

Gerd B. McGwire^§, Robert P. Becker^¶, and Randal A. Skidgel^§

From the  Laboratory of Peptide Research and the Departments of ^§ Pharmacology,  Anesthesiology, and ^¶ Anatomy and Cell Biology, University of Illinois College of Medicine, Chicago, Illinois 60612

Carboxypeptidase M, a glycosylphosphatidylinositol-anchored membrane glycoprotein, is highly expressed in Madin-Darby canine kidney (MDCK) cells, where it was previously shown that the glycosylphosphatidylinositol anchor and N-linked carbohydrate are apical targeting signals. Here, we show that carboxypeptidase M has an unusual, non-polarized distribution, with up to 44% on the basolateral domain of polarized MDCK cells grown on semipermeable inserts. Alkaline phosphatase, as well as five other glycosylphosphatidylinositol-anchored proteins, and transmembrane -glutamyl transpeptidase exhibited the expected apical localization. Basolateral carboxypeptidase M was readily released by exogenous phosphatidylinositol-specific phospholipase C, showing it is glycosylphosphatidylinositol-anchored, whereas apical carboxypeptidase M was more resistant to release. In contrast, the spontaneous release of carboxypeptidase M into the medium was much higher on the apical than the basolateral domain. In pulse-chase studies, newly synthesized carboxypeptidase M arrived in equal amounts within 30 min on both domains, indicating direct sorting. After 4-8 h of chase, the steady-state distribution was attained, possibly due to transcytosis from the basolateral to the apical domain. These data suggest the presence of a unique basolateral targeting signal in carboxypeptidase M that competes with its apical targeting signals, resulting in a non-polarized distribution in MDCK cells.

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