Poly-ubiquitylation of α-tubulin at K304 is required for flagellar disassembly in Chlamydomonas

Abstract

Cilia/flagella are structurally conserved and dynamic organelles; their assembly and disassembly are coordinated with the cell cycle and cell differentiation. Several post-translational modifications, including acetylation, methylation, phosphorylation and ubiquitylation, participate in ciliary disassembly. However, the detailed mechanism and the role of ubiquitylation in ciliary disassembly are unclear. This study identified 20 proteins that were ubiquitylated in shortening flagella of Chlamydomonas. α-Tubulin was the most abundant ubiquitylated protein and it was labeled with K63 poly-ubiquitin chains primarily at K304. Expression of an α-tubulin mutant (K304R), that could not be ubiquitylated, decreased the rate of flagellar disassembly and resulted in an enrichment of the mutant form in the axoneme, suggesting that ubiquitylation of α-tubulin is required for the normal kinetics of axonemal disassembly. Immunoprecipitation and GST pull-down assays demonstrated that the retrograde IFT protein, IFT139, interacted with a variety of ubiquitylated proteins, including α-tubulin, suggesting that IFT-A was responsible for transporting ubiquitylated proteins out of the flagella. Our data suggested an important role for ubiquitylation and retrograde IFT in ciliary disassembly.