Rab8a is necessary for the trafficking of polycystin-2 into the primary cilium. Stably transfected LLC-PK₁ cells inducibly synthesizing a fusion protein between EGFP and polycystin-2 (1–703) were microinjected with an expression plasmid encoding a fusion protein between RFP and the dominant-negative Rab8a T22N mutant protein. The cell marked by an asterisk synthesizes both Rab8a T22N (a) and polycystin-2 (1–703) (b), but its primary cilium (c and d, arrowhead) does not contain polycystin-2 (1–703). In other cells not synthesizing Rab8a T22N, the primary cilium is positive for polycystin-2 (1–703) (b–d, arrow). Bar, 20 µm.