Revaluating the evolution of the Toc and Tic protein translocons

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The origin of the plastid from a cyanobacterial endosymbiont necessitated the establishment of specialized molecular machines (translocons) to facilitate the import of nuclear-encoded proteins into the organelle. To improve our understanding of the evolution of the translocons at the outer and inner envelope membrane of chloroplasts (Toc and Tic, respectively), we critically reassess the prevalent notion that their subunits have a function exclusive to protein import. We propose that many translocon components are multifunctional, conserving ancestral pre-endosymbiotic properties that predate their recruitment into the primitive translocon (putatively composed of subunits Toc34, Toc75 and Tic110 and associated chaperones). Multifunctionality seems to be a hallmark of the Tic complex, in which protein import is integrated with a broad array of plastid processes.