Protein transport in the connecting cilium of a photoreceptor cell: Modeling the effects of bidirectional protein transitions between the diffusion-driven and motor-driven kinetic states

Physics of protein transport through the connecting cilium (CC) of a photoreceptor cell is a long-standing question in cellular biology. There is evidence implicating both molecular motor-driven and diffusion-driven modes of intracellular transport. Based on available experimental clues, this paper develops a new model for intraflagellar transport (IFT) of proteins synthesized in the inner segment and transported through the CC to the outer segment of a photoreceptor cell. The model accounts for the competition between two modes of protein transport: molecular motor-driven transport and diffusion. The obtained solutions made it possible to calculate how the number of protein molecules transported through the CC at a given time depends on their diffusivity. Modeling results were compared with published experimental estimates, and conclusions about possible contributions of diffusion to IFT were made.