Organelle Size and Quality Control in Chlamydomonas Reinhardtii
Organelles represent dynamic entities within the cell that undergo differential protein quality control regulation and size control changes depending on fluctuations in cellular conditions, environmental cues, and cell stage. In both halves of my study I used the unicellular algae, Chlamydomonas reinhardtii, to study these two facets of biology using two of its organelles: the chloroplast and the flagellum. In the first half of this study, I investigated a signaling pathway known as the chloroplast Unfolded Protein Response (cpUPR). In response to proteotoxic stress, chloroplasts communicate with the nuclear gene expression system through a chloroplast unfolded protein response (cpUPR). We isolated mutants that disrupt cpUPR signaling and identified a gene encoding a previously uncharacterized cytoplasmic protein kinase, termed Mars1—for mutant affected in chloroplast-to-nucleus retrograde signaling—as the first known component in cpUPR signal transmission. Lack of cpUPR induction in MARS1 mutant cells impaired their ability to cope with chloroplast stress, including exposure to excessive light. In the second half, I studied organelle size control using regenerating flagella. We discovered a loss-of-function mutation in a gene that leads to shortened flagella. This gene, which encodes a Chlamydomonas ortholog of Crescerin, corresponds to the previously unknown short flagella gene SHF1. Crescerin/SHF1 has been identified as a cilia-specific TOG-domain array protein that binds to tubulin. In this mutant, we found that flagellar regeneration occurs with the same initial kinetics as wild-type cells, but plateaus at a shorter length. We place this finding in the larger context of tubulin dynamics by suggesting that this TOG-domain array protein is necessary to efficiently and preemptively increase tubulin levels to offset decreasing IFT cargo at the tip as flagellar assembly progresses.