ARC3 required for constriction

Abstract

Chloroplast division requires the formation of a stromal Z ring composed of two tubulin-like FtsZ homologues, FtsZ1 and FtsZ2. This ring is linked to the inner envelope membrane through ARC6 and further connected to the outer envelope membranes through PLASTID DIVISION proteins and to a cytosolic plastid division ring containing dynamin-like ARC5. The correct positioning of the Z ring depends on a chloroplast version of the Min system in which ARC3 has a key role as a negative regulator of Z ring formation. ARC3 displays a diffuse localization at non-division sites where it probably prevents Z ring formation. However, ARC3 also localizes to the Z ring itself, but the function of this subpopulation was unclear.

The researchers show that ARC3 physically interacts with ARC6, and that this interaction depends on its membrane occupation and recognition nexus (MORN) domain with which it was known to bind to the paralogue of ARC6 (PARC6). Indeed, localization of ARC3 in arc6 parc6 and arc6 mutant background suggests that there is some functional overlap between ARC6 and PARC6 in recruiting ARC3 to the Z ring. Expression of ARC3 with a deleted MORN domain abolishes filament assembly of FtsZ1 in a heterologous yeast system, providing evidence for the autoinhibitory function of this domain. Co-expression of ARC6 with ARC3 inhibited FtsZ1 filament formation, presumably due to interaction with the MORN domain, which may release the autoinhibitory function similar to PARC6. However, in contrast to PARC6, ARC6 has a J-like domain that prevents interaction with ARC3 in planta. Indeed, deletion of the J-like domain of ARC6 led to reduced FtsZ1 assembly and chloroplast division defects, similar to plants overexpressing ARC3. This is not the case in the arc3 mutant background, suggesting that inhibition of FtsZ1 assembly depends on ARC3. Intriguingly, co-expression of ARC3 and ARC6 in the heterologous system Pichia pastoris led to enhanced Z ring constriction dynamics, suggesting that ARC3 function is not only negatively controlled by ARC6 within the Z ring but is also beneficial for constriction dynamics.