Length regulation of multiple flagella that self-assemble from a shared pool of components

Abstract

The single-celled green algae Chlamydomonas reinhardtii with its two flagella—microtubule-based structures of equal and constant lengths—is the canonical model organism for studying size control of organelles. Experiments have identified motor-driven transport of tubulin to the flagella tips as a key component of their length control. Here we consider a class of models whose key assumption is that proteins responsible for the intraflagellar transport (IFT) of tubulin are present in limiting amounts. We show that the limiting-pool assumption is insufficient to describe the results of severing experiments, in which a flagellum is regenerated after it has been severed. Next, we consider an extension of the limiting-pool model that incorporates proteins that depolymerize microtubules. We show that this ‘active disassembly’ model of flagellar length control explains in quantitative detail the results of severing experiments and use it to make predictions that can be tested in experiments.

Functional exploration of heterotrimeric kinesin-II in IFT and ciliary length control in Chlamydomonas

“…The eukaryotic cilium is a model system for probing the phenomenon of organellar size control and equilibration ( Chan and Marshall, 2012 ). In Chlamydomonas , several models have been proposed to explain ciliary length control ( Bertiaux et al, 2018b ; Fai et al, 2019 ; Hendel et al, 2018 ; Ludington et al, 2015 ; Ma et al, 2020 ; Marshall and Rosenbaum, 2001 ; Patra et al, 2020 ; Wemmer et al, 2020 ). Our experiments show that a motor with ~2.8-fold reduction in speed results in a small change in ciliary length (~15% shorter).…”

Control of filament length by a depolymerizing gradient

“…Depending on the details of the transport, the amount of protein accumulated at the tip, and therefore its depolymerizing activity, will either depend linearly or exponentially on the filament length [20,24]. Most importantly, in the presence of multiple filaments drawing from the same pool of depolymerizing proteins, the concentration at the tip of each filament will only depend on the length of that particular filament, thereby enabling simultaneous length control of multiple filaments [18]. This is in contrast to length control by a finite monomer pool in which case only the total length of all the filaments is precisely controlled, while each individual filament’s length can vary wildly [12].…”

“…In particular, we do not take into account the length dependence of k + that can arise due to a limited pool of motors that transport monomers to the tip of the flagellum (Figure 1), which was analyzed in ref. [18]. There it was shown while this feature of the assembly dynamics is important for getting the regeneration dynamics to agree with experiments in Chlamy, but it does not provide length control for multiple flagella.…”

“…The two models investigated in the previous subsection both describe the stochastic assembly of filaments that produce a well-defined length, and unlike the limited subunit-pool model [12,18] they can simultaneously regulate the lengths of any number of filaments. This is because the dynamics of filament assembly are independent of one another.…”

“…The mechanism of length control of the two Chlamy flagella that has thus far received the most attention is one that focuses on length dependent assembly [16,17]. Recently we have shown that length dependent assembly can arise from a limited and shared pool of motors that transport tubulin to the flagellar tip, where assembly occurs [18]. In this case, though, the assembly rate for the two flagella is the same and depends on the sum of their lengths.…”