Huang et al. report that the ubiquitination system has an integral function in disassembling cilia and flagella.
Cilia and flagella are whip-like membrane protrusions that propel cells around and are implicated in many human disorders, collectively known as ciliopathies. New research demonstrates that they also function as sensory and signal transducers. Cilia and flagella grow and shrink during the cell cycle, becoming completely resorbed before the cell divides. Whereas the mechanism by which cilia assemble has been well studied, how they disassemble remains largely unknown.
Huang et al. isolated flagella from the green alga Chlamydomonas reinhardtii and found that they contained a completely functional ubiquitination system, including free ubiquitin and enzymes necessary for its conjugation. Ubiquitination activity, and the abundance of ubiquitinated proteins increased in response to cues stimulating flagellar disassembly. However, though the group found an active ubiquitination system, there was no evidence of a flagellar-localized proteasome.
The researchers also found that the targets of ubiquitination include α-tubulin and the cation channel CrPKD2, an algal homologue of a gene mutated in the ciliopathy polycystic kidney disease. The functional consequence of CrPKD2 ubiquitination is not known, but the authors hypothesize that it facilitates recycling. In lieu of degrading flagellar components and making them anew, some proteins may be sent back to the distal tip for reassembly. It seems these algae may be green in more ways than one. EC
