IFT88 (green) is found in mouse photoreceptor cilia (red).

To understand vision in vertebrates, one might look to worms and algae, according to recent results. On page 103, Pazour et al. show that a conserved protein transport mechanism in the flagellae of algae and worms is also necessary for development and maintenance of mammalian photoreceptors.

Assembly and maintenance of motor and sensory cilia in worms and algae requires the transport of proteins via intraflagellar transport (IFT). The outer segment (OS) of a vertebrate photoreceptor, itself a modified cilium, is formed by transporting membrane and opsin through a connecting cilium that provides a link between the photoreceptor inner (metabolic) and outer (photoreceptive) segments. Photopigment molecules and phototransduction proteins must pass through this connecting cilium to replace components of the OS that have been degraded.

Now, it appears that the mechanism used to transport these photoreceptor molecules is conserved throughout eukaryotes. Pazour et al. identified mouse and human homologues of several proteins that form a complex required for IFT in algae. The vertebrate proteins form a complex of similar density to that of the algal complex and localize to the connecting cilia of photoreceptors.

Defects in one of these proteins, IFT88, leads to abnormal development of the mouse photoreceptor OS. Retinal degeneration occurs via apoptotic death of photoreceptor cells, presumably because necessary OScomponents are not transported. Mutations in the human IFT genes are candidates for causing degenerative vision disorders such as retinitis pigmentosa. ▪