Newly synthesized polycystin‐1 takes different trafficking pathways to the apical and ciliary membranes.

Mutations in the genes encoding polycystin‐1 (PC1) and polycystin 2 (PC2) cause autosomal dominant polycystic kidney disease. These transmembrane proteins colocalize in the primary cilia of renal epithelial cells, where they may participate in sensory processes. PC1 is also found in the apical membrane when expressed in cultured epithelial cells. PC1 undergoes autocatalytic cleavage, producing an extracellular N‐terminal fragment that remains noncovalently attached to the transmembrane C‐terminus. Exposing cells to alkaline solutions elutes the N‐terminal fragment while the C‐terminal fragment is retained in the cell membrane. Utilizing this observation, we developed a "strip‐recovery" synchronization protocol to study PC1 trafficking in polarized LLC‐PK1 renal epithelial cells. Following alkaline strip, a new cohort of PC1 repopulates the cilia within 30 minutes, while apical delivery of PC1 was not detectable until 3 hours. Brefeldin A (BFA) blocked apical PC1 delivery, while ciliary delivery of PC1 was BFA insensitive. Incubating cells at 20°C to block trafficking out of the trans‐Golgi network also inhibits apical but not ciliary delivery. These results suggest that newly synthesized PC1 takes distinct pathways to the ciliary and apical membranes. Ciliary PC1 appears to by‐pass BFA sensitive Golgi compartments, while apical delivery of PC1 traverses these compartments. The polycystin‐1 protein undergoes an autocatalytic cleavage that releases its large extracellular N‐terminal domain, which remains noncovalently attached to its transmembrane domains. Exposing cells to alkaline pH strips off the N‐terminal domain, and this property was employed in an experiment designed to examine the post‐synthetic trafficking of polycystin‐1. While the apical membrane pool of protein passes through the Golgi complex, the ciliary pool appears to pursue a Golgi‐bypass pathway. [ABSTRACT FROM AUTHOR]