Perlaza, K. | Search bioRxiv

bioRxiv2022

Role of intraflagellar transport in transcriptional control during flagellar regeneration in Chlamydomonas

Biosynthesis of organelle precursors is a central part of the organelle size control problem, but what systems are required to control precursor production? Genes encoding flagellar proteins are upregulated during flagellar regeneration in Chlamydomonas, and this upregulation is critical for flagella to reach their final length, but it not known how the cell triggers these genes during regeneration. Here we present two models based on transcriptional repressor that is either produced in the flagellum, or else is produced in the cell body and sequestered in the growing flagellum. We show that both models lead to stable flagellar length control, can reproduce the observed dynamics of gene expression, and are consistent with the effects of protein synthesis inhibitors on gene expression. The two models make opposite predictions regarding the effect of mutations that block intraflagellar transport (IFT). Using quantitative measurements of gene expression, we show that gene expression during flagellar regeneration is greatly reduced in mutations of FLA3, FLA8, and FLA10, which encode the three components of the heterotrimer kinesin-2 that drives IFT. This result is consistent with the predictions of the model in which a repressor is sequestered in the flagellum by IFT. In contrast to the effects of IFT mutants, we find that inhibiting axonemal assembly has much less effect on gene expression, suggesting that transport is more important than axonemal assembly. The repressor sequestration model allows precursor production to occur when flagella are growing rapidly, representing a form of derivative control.

cell biology↗

bioRxiv2021

The short flagella 1 (SHF1) gene in Chlamydomonas encodes a Crescerin TOG-domain protein required for late stages of flagellar growth.

Length control of flagella represents a simple and tractable system to investigate the dynamics of organelle size. Models for flagellar length control in the model organism, Chlamydomonas reinhardtii have focused on the length-dependence of the intraflagellar transport (IFT) system which manages the delivery and removal of axonemal subunits at the tip of the flagella. One of these cargoes, tubulin, is the major axonemal subunit, and its frequency of arrival at the tip plays a central role in size control models. However, the mechanisms determining tubulin dynamics at the tip are still poorly understood. We discovered a loss-of-function mutation that leads to shortened flagella, and found that this was an allele of a previously described gene, SHF1, whose molecular identity had not previously been determined. We found that SHF1 encodes a Chlamydomonas ortholog of Crescerin, previously identified as a cilia-specific TOG-domain array protein that can bind tubulin via its TOG domains and increase tubulin polymerization rates. In this mutant, flagellar regeneration occurs with the same initial kinetics as wild-type cells, but plateaus at a shorter length. Using a computational model in which the flagellar microtubules are represented by a differential equation for flagellar length combined with a stochastic model for cytoplasmic microtubule dynamics, we found that our experimental results are best described by a model in which Crescerin/SHF1 binds tubulin dimers in the cytoplasm and transports them into the flagellum. We suggest that this TOG-domain protein is necessary to efficiently and preemptively increase intra-flagella tubulin levels to offset decreasing IFT cargo at the tip as flagellar assembly progresses.

cell biology↗