Single-Molecule Tracking Reveals Complex Motility of Transmembrane Proteins in the Chemosensory Cilia of C. elegans

Abstract

Cilia are vital for the cell's ability to sense its environment and rely on a process called intra flagellar transport (IFT) for their development, maintenance and function in signal-transduction. In IFT, motor proteins transport ciliary components along the polarized microtubule axoneme of the cilium. Among the cargoes of IFT are transmembrane proteins involved in signal transduction. As a model system, we study C. elegans chemosensory cilia, which we study using fluorescence microscopy with single-molecule sensitivity. First, we demonstrate that IFT machinery and ciliary components, including TRPV transmembrane channel protein OCR-2 are redistributed away from the ciliary tip upon external chemical stimulation, in a robust, extensive and reversible way. To elucidate the dynamics underlying this dramatic protein redistribution, we performed single-molecule imaging of OCR-2 in live C. elegans. Advanced analysis of the single-molecule trajectories shows that, in dendrite and transition zone, active transport is the prevailing motility mode of OCR-2. In the proximal and distal segments, however, motility is a much more complex, location-specific interplay between active transport, normal diffusion and sub diffusion. At the tip, confinement of the membrane proteins plays an important role. Together, our data and analysis demonstrate an intricate interplay between modes of transportation that ensure the proper ciliary distribution of OCR-2. These insights in the dynamics of cellular signal-transduction contributes to a wider understanding of IFT dynamics and to cilia as chemosensory organelles.