Author:

Abstract:

Proteome analysis of isolated cilia revealed hundreds of proteins essentially dedicated to cilia organization and function. However, it is still not known how membrane proteins are targeted to the cilium. They are presumably synthesized in the ER and then transported to the cilia, but how they reach the ciliary membrane is largely unknown. Unexpectedly, we recently identified a protein from the early secretory pathway, Rer1p, to be crucial in cilia development and cell polarity. Interestingly, Rer1p operates in the quality control of membrane proteins that occurs between the ER and Golgi complex. Therefore, Rer1p represents the link between the early secretory pathway compartments and a distant organelle such as cilia and might reveal the mechanisms behind sorting of membrane proteins destined for cilia. Here we demonstrate that Rer1p has specifically high expression in ciliated organs of zebrafish, such as olfactory pit, otic vesicle, neuromasts of the lateral line and pronephros. Moreover, MO-mediated knockdown of its gene in zebrafish embryos results in a typical phenotype found in mutants with defective cilia. For instance, zebrafish morphants present with bent body axis, laterality defects and impaired development of lateral line organ. Strikingly, downregulation of Rer1p was sufficient to cause embryonic deafness, pointing to essential role of this protein in proper development and function of the inner ear. Importantly, some of these features are commonly observed in patients suffering from cilia-related disorders, known as ciliopathies. In addition, we observed that Rer1p knockdown in LLC-PK1-CL4 (porcine kidney epithelial) cell line severely reduces cilia length, while it increases the amount of acetylated tubulin, a stable form of tubulin found in cilia. Similar increase in acetylation is observed throughout the development of ciliated cells in zebrafish. This already offers a first hint into the possible mechanism of Rer1p involvement in ciliogenesis. Here we describe for the first time the involvement of a quality control mechanism of early biosynthetic compartments in the morphogenesis or maintenance of functional cilia.