Hierarchal order in the formation of chloroplast division machinery in the red alga Cyanidioschyzon merolae

The peptidoglycan synthase PBP interacts with PLASTID DIVISION2 to promote chloroplast division in Physcomitrium patens

The peptidoglycan (PG) layer, a core component of the bacterial cell wall, has been retained in the Physcomitrium patens chloroplasts. The PG layer entirely encompasses the P. patens chloroplast, including the division site, but how PG biosynthesis cooperates with the constriction of two envelope membranes at the chloroplast division site remains elusive. Here, focusing on the PG synthase penicillin-binding protein (PBP), we performed cytological and molecular analyses to dissect the mechanism of chloroplast division in P. patens. We showed that PBP, acting in the final step of PG biosynthesis, is likely a chloroplast inner envelope protein that can aggregate at mid-chloroplasts during chloroplast division. Physcomitrium patens had five orthologs of PLASTID DIVISION2 (PDV2), an outer envelope component of the chloroplast division complex. Our data indicated that PpPDV2 proteins interact with PpPBP and are responsible for recruiting PpPBP to the chloroplast division site, in addition to PpDRP5B. Furthermore, we found that PBP deletion and carbenicillin application restrain constriction of the chloroplast division complex, rather than its assembly. This work provides direct molecular evidence for a link between chloroplast division of P. patens and PG biosynthesis and indicates that PG biosynthesis is required for the constriction of the chloroplast division apparatus in P. patens.

Insights into the Mechanisms of Chloroplast Division

The endosymbiosis of a free-living cyanobacterium into an ancestral eukaryote led to the evolution of the chloroplast (plastid) more than one billion years ago. Given their independent origins, plastid proliferation is restricted to the binary fission of pre-existing plastids within a cell. In the last 25 years, the structure of the supramolecular machinery regulating plastid division has been discovered, and some of its component proteins identified. More recently, isolated plastid-division machineries have been examined to elucidate their structural and mechanistic details. Furthermore, complex studies have revealed how the plastid-division machinery morphologically transforms during plastid division, and which of its component proteins play a critical role in generating the contractile force. Identifying the three-dimensional structures and putative functional domains of the component proteins has given us hints about the mechanisms driving the machinery. Surprisingly, the mechanisms driving plastid division resemble those of mitochondrial division, indicating that these division machineries likely developed from the same evolutionary origin, providing a key insight into how endosymbiotic organelles were established. These findings have opened new avenues of research into organelle proliferation mechanisms and the evolution of organelles.