PARC6, a novel chloroplast division factor, influences FtsZ assembly and is required for recruitment of PDV1 during chloroplast division in Arabidopsis.

Chloroplast division in plant cells is accomplished through the coordinated action of the tubulin-like FtsZ ring inside the organelle and the dynamin-like ARC5 ring outside the organelle. This coordination is facilitated by ARC6, an inner envelope protein required for both assembly of FtsZ and recruitment of ARC5. Recently, we showed that ARC6 specifies the mid-plastid positioning of the outer envelope proteins PDV1 and PDV2, which have parallel functions in dynamin recruitment. PDV2 positioning involves direct ARC6-PDV2 interaction, but PDV1 and ARC6 do not interact indicating that an additional factor functions downstream of ARC6 to position PDV1. Here, we show that PARC6 (paralog of ARC6), an ARC6-like protein unique to vascular plants, fulfills this role. Like ARC6, PARC6 is an inner envelope protein with its N-terminus exposed to the stroma and Arabidopsis parc6 mutants exhibit defects of chloroplast and FtsZ filament morphology. However, whereas ARC6 promotes FtsZ assembly, PARC6 appears to inhibit FtsZ assembly, suggesting that ARC6 and PARC6 function as antagonistic regulators of FtsZ dynamics. The FtsZ inhibitory activity of PARC6 may involve its interaction with the FtsZ-positioning factor ARC3. A PARC6-GFP fusion protein localizes both to the mid-plastid and to a single spot at one pole, reminiscent of the localization of ARC3, PDV1 and ARC5. Although PARC6 localizes PDV1, it is not required for PDV2 localization or ARC5 recruitment. Our findings indicate that PARC6, like ARC6, plays a role in coordinating the internal and external components of the chloroplast division complex, but that PARC6 has evolved distinct functions in the division process.