title: Redefining the metabolic continuity of chloroplasts and ER.
abstract: As a hub for plant metabolism, plastids extensively exchange metabolites with the extraplastid environment. For polar metabolites, membrane transporters mediate this exchange, but for many plastid-synthesized nonpolar compounds, such transporters remain elusive. Here, we discuss recent data from transorganellar complementation studies that demonstrate that enzymes in one organelle can directly access nonpolar metabolites from a companion organelle. We propose that a mechanism, based on hemifused-membranes at plastid-endoplasmic reticulum (ER) contact sites, facilitates interorganellar interactions and allows enzymes direct, transporter-independent access to a range of nonpolar compounds in both organelle membranes. In a wider context, interorganellar metabolism at hemifusion interfaces would allow evolution of membrane-spanning pathways for the many thousands of nonpolar metabolites in the plant kingdom to be uncoupled from coevolution with nonpolar metabolite transporters.
authors.affiliation: Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA.
authors.forename: Payam
authors.initials: P
authors.lastname: Mehrshahi
authors.affiliation: Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA.
authors.forename: Cassandra
authors.initials: C
authors.lastname: Johnny
authors.affiliation: Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, MI 48824, USA. Electronic address: dellapen@msu.edu.
authors.forename: Dean
authors.initials: D
authors.lastname: DellaPenna
mesh_terms: D005075:Biological Evolution; D002736:Chloroplasts / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D008561:Membrane Fusion; D008954:Models, Biological; D059828:Plant Cells / Q000378:metabolism
keywords: PLAM; hemifusion; interorganellar; nonpolar; transorganellar complementation