A Novel zf-MYND Protein, CHB-3, Mediates Guanylyl Cyclase Localization to Sensory Cilia and Controls Body Size of Caenorhabditis elegans

Abstract

Cilia are important sensory organelles, which are thought to be essential regulators of numerous signaling pathways. In Caenorhabditis elegans, defects in sensory cilium formation result in a small-body phenotype, suggesting the role of sensory cilia in body size determination. Previous analyses suggest that lack of normal cilia causes the small-body phenotype through the activation of a signaling pathway which consists of the EGL-4 cGMP-dependent protein kinase and the GCY-12 receptor-type guanylyl cyclase. By genetic suppressor screening of the small-body phenotype of a cilium defective mutant, we identified a chb-3 gene. Genetic analyses placed chb-3 in the same pathway as egl-4 and gcy-12 and upstream of egl-4. chb-3 encodes a novel protein, with a zf-MYND motif and ankyrin repeats, that is highly conserved from worm to human. In chb-3 mutants, GCY-12 guanylyl cyclase visualized by tagged GFP (GCY-12::GFP) fails to localize to sensory cilia properly and accumulates in cell bodies. Our analyses suggest that decreased GCY-12 levels in the cilia of chb-3 mutants may cause the suppression of the small-body phenotype of a cilium defective mutant. By observing the transport of GCY-12::GFP particles along the dendrites to the cilia in sensory neurons, we found that the velocities and the frequencies of the particle movement are decreased in chb-3 mutant animals. How membrane proteins are trafficked to cilia has been the focus of extensive studies in vertebrates and invertebrates, although only a few of the relevant proteins have been identified. Our study defines a new regulator, CHB-3, in the trafficking process and also shows the importance of ciliary targeting of the signaling molecule, GCY-12, in sensory-dependent body size regulation in C. elegans. Given that CHB-3 is highly conserved in mammal, a similar system may be used in the trafficking of signaling proteins to the cilia of other species.

Author Summary

Caenorhabditis elegans is a 1–2 mm long nematode. Its body size is controlled by sensory inputs; some mutants with defects in sensory perception grow into small size (20%–30% decrease in body volume), although the animals seem to feed normally. The EGL-4 cGMP-dependent protein kinase and the GCY-12 guanylyl cyclase act in sensory neurons to control body size downstream of sensory inputs. GCY-12 is localized to cilia, antenna-like cellular structures of sensory neurons. In the cilia, a number of signaling molecules are localized. Dysfunction of cilia is known to cause several human disorders such as Bardet-Biedl syndrome, illustrating the importance of these organelles. In this study, we identified a novel protein, CHB-3, involved in sensory-dependent body size regulation. Our analyses suggest that CHB-3 protein regulates the trafficking of GCY-12 from the cell bodies to the cilia. Without CHB-3 protein, GCY-12 fails to localize to cilia and body size is not controlled properly. Thus, the cilia are a special place for sensory information processing in body size regulation. Our analyses identified CHB-3 as a novel trafficking regulator of ciliary protein(s).