Osteryoung, K. W. | Search bioRxiv

bioRxiv2024

The J-Like Protein ARC6 Regulates Chloroplast FtsZ-Ring Assembly through Fine-tuning ARC3 Activity in Arabidopsis

Chloroplast division is initiated by the establishment of the stromal FtsZ ring (Z ring). Assembly and positioning of the Z ring are governed by the chloroplast Min system, which inhibits Z-ring formation everywhere but the middle of the chloroplast. ACCUMULATION AND REPLICATION OF CHLOROPLASTS3 (ARC3), the core component of this system, is a direct inhibitor of Z-ring assembly. Regulation of ARC3 activity is vital thus for chloroplast division. Here, we report that ARC6, which localizes on the chloroplast inner envelope membrane, interacts with ARC3 and acts upstream of ARC3 during chloroplast division. We show that the C-terminal MORN domain of ARC3, demonstrated previously to prevent ARC3-FtsZ interaction, binds to the J-like domain (JLD) of ARC6, enabling full-length ARC3 to interact with FtsZ proteins and activating the inhibitory activity of ARC3 on the assembly of FtsZ filaments. Overexpression of a JLD-deleted version of ARC6 causes disruption of Z-ring formation in an ARC3-dependent manner. Finally, we reveal that ARC6 recruits ARC3 to the middle of the chloroplast. Our findings suggest a model whereby ARC6 regulates the assembly and positioning of the Z ring through fine-tuning the inhibitory activity of ARC3 at the chloroplast division site. One Sentence SummaryThe chloroplast membrane protein ARC6 recruits ARC3 to the chloroplast division site and regulates the assembly of the FtsZ ring by fine-tuning ARC3 activity through its J-like domain. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (https://academic.oup.com/plcell/pages/General-Instructions) is: Cheng Chen (cgchen@sjtu.edu.cn).

plant biology↗

bioRxiv2018

Protein Gradients on the Nucleoid Position the Carbon-fixing Organelles of Cyanobacteria

In BriefWe describe a novel positioning system that utilizes the cyanobacterial nucleoid to segregate, transport and equidistantly position the carbon-fixation machinery (carboxysomes).\n\nSummaryCarboxysomes are protein-based bacterial organelles that encapsulate a key enzyme of the Calvin-Benson-Bassham cycle. Previous work has implicated a ParA-like protein (hereafter McdA) as important for spatially organizing carboxysomes along the longitudinal axis of the model cyanobacterium Synechococcus elongatus PCC 7942. Yet, how self-organization of McdA emerges and contributes to carboxysome positioning is unknown. Here, we show that a small protein, termed McdB, localizes to carboxysomes through interactions with carboxysome shell proteins to drive emergent oscillatory patterning of McdA on the nucleoid. Our results demonstrate that McdB directly interacts to stimulate McdA ATPase activity, and indicate that carboxysome-dependent McdA depletion zone formation on the nucleoid is required for directed motion of carboxysomes towards increased concentrations of McdA. We propose that McdA and McdB are a new class of self-organizing proteins that follow a Brownian-ratchet mechanism, challenging the cytoskeletal model of organelle transport, for equidistant positioning of carboxysomes in cyanobacteria. These results have broader implications for understanding spatial organization of protein mega-complexes and organelles in bacteria more broadly.

microbiology↗