Role of MCD1 in Positioning the Chloroplast Division Site

Chen et al. investigate the mechanism of MCD1’s action in chloroplast Z-ring positioning in dividing chloroplasts. https://doi.org/10.1105/tpc.18.00189

Background: Chloroplasts are specialized photosynthetic organelles in plants that evolved from an ancient photosynthesizing cyanobacterium through endosymbiosis. Like their ancestors, chloroplasts replicate by binary fission, which is initiated by the bacteria-like cytoskeletal FtsZ ring (Z-ring) at mid-chloroplast. A “Min system” complex, composed of the three proteins MinC, MinD, and MinE, widely exists in bacteria to regulate Z-ring formation at mid-cell. However, in addition to this bacteria-like Min system, the plant-specific membrane protein MCD1 is also involved in chloroplast Z-ring positioning in plants, but the role of it in Z-ring placement at mid-chloroplast is unknown.

Question: What is the membrane topology of Arabidopsis MCD1? Since MCD1 functions in Z-ring positioning, we asked whether MCD1 directly interacts with FtsZ proteins.  What’s the mechanism of MCD1’s action? And what are the functional relationships of MCD1 with the bacteria-like Min system in plants, which includes ARC3 (a replacement of bacterial MinC), MinD1, and MinE1?

Findings: We found that MCD1 is a bitopic inner membrane protein whose C terminus faces the chloroplast stroma. Interaction analysis showed that MCD1 recognizes membrane-tethered FtsZ filaments through a direct interaction with ARC6 rather than interacting with FtsZ proteins per se. These results are consistent with the in vivo observation that ARC6 influences the localization of MCD1 to membrane-tethered FtsZ filaments. Additionally, we found that MCD1 is required for the regulation of Z-ring positioning by ARC3, MinD1, and MinE1. Together, our findings indicate that MCD1 protein is part of the chloroplast Min system that recognizes membrane-tethered FtsZ filaments through interacting with ARC6 during chloroplast division-ring positioning.

Next steps: Our work shows that the emergence of MCD1 in plants likely facilitated the precise localization of the ARC3 and MinD1 complex to the membrane-tethered FtsZ filaments. A logical next step is to investigate how MinE1 temporally and spatially restricts the distribution of MCD1 on the inner membrane.

Li Chen, Bing Sun, Wei Gao, Qi-yang Zhang, Huan Yuan, Min Zhang. (2018). MCD1 Associates with FtsZ Filaments via the Membrane-Tethering Protein ARC6 to Guide Chloroplast Division. Plant Cell Aug 2018, 30: 1807-1823. https://doi.org/10.1105/tpc.18.00189