CRL4WDR1 Controls Polo-like Kinase Protein Abundance to Promote Bilobe Duplication, Basal Body Segregation and Flagellum Attachment in Trypanosoma brucei

The Polo-like kinase homolog in Trypanosoma brucei, TbPLK, plays essential roles in basal body segregation, flagellum attachment and cytokinesis. The level of TbPLK protein is tightly controlled, but the underlying mechanism remains elusive. Here, we report a Cullin-RING ubiquitin ligase composed of Cullin4, the DNA damage-binding protein 1 homolog TbDDB1 and a WD40-repeat protein WDR1 that controls TbPLK abundance in the basal body and the bilobe. WDR1, through its C-terminal domain, interacts with the PEST motif in TbPLK and, through its N-terminal WD40 motif, binds to TbDDB1. Depletion of WDR1 inhibits bilobe duplication and basal body segregation, disrupts the assembly of the new flagellum attachment zone filament and detaches the new flagellum. Consistent with its role in TbPLK degradation, depletion of WDR1 causes excessive accumulation of TbPLK in the basal body and the bilobe, leading to continuous phosphorylation of TbCentrin2 in the bilobe at late cell cycle stages. Together, these results identify a novel WD40-repeat protein as a TbPLK receptor in the Cullin4-DDB1 ubiquitin ligase complex for degrading TbPLK in the basal body and the bilobe after the G1/S cell cycle transition, thereby promoting bilobe duplication, basal body separation and flagellum-cell body adhesion.
Author Summary African trypanosomes are the causative agents of sleeping sickness in humans and nagana in cattle in sub-Saharan Africa. These unicellular parasites possess a flagellum, which is a long, lash-like appendage that is protruded from the cell body and serves for locomotion of the parasites. The flagellum is attached to the cell body via a specialized structure termed flagellum attachment zone (FAZ) filament. Assembly of the FAZ filament is necessary to maintain flagellum-cell body attachment, which is required for maintaining normal cell shape and for ensuring faithful cell division. A key regulator of flagellum-cell body attachment is the Polo-like kinase (PLK), an evolutionarily conserved protein kinase that regulates many downstream effectors. The level of PLK appears to be tightly controlled, but the underlying mechanism is still elusive. Here we report a novel protein that controls PLK abundance by targeting PLK for degradation. Depletion of this novel protein causes excessive accumulation of PLK and disrupts FAZ filament assembly, leading to flagellum detachment and defective cell division. These results discover the molecular mechanism underlying the control of PLK protein abundance in trypanosomes.