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New mechanistic insights into pre‐protein transport across the second outermost plastid membrane of diatoms
Author(s) -
Hempel Franziska,
Felsner Gregor,
Maier Uwe G.
Publication year - 2010
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/j.1365-2958.2010.07142.x
Subject(s) - plastid , biology , phaeodactylum tricornutum , translocon , endoplasmic reticulum , ubiquitin ligase , endoplasmic reticulum associated protein degradation , microbiology and biotechnology , endosymbiosis , organelle , ubiquitin , biochemistry , membrane protein , membrane , diatom , botany , gene , unfolded protein response , chloroplast
Summary Chromalveolates like the diatom Phaeodactylum tricornutum arose through the uptake of a red alga by a phagotrophic protist, a process termed secondary endosymbiosis. In consequence, the plastids are surrounded by two additional membranes compared with primary plastids. This plastid morphology poses additional barriers for plastid‐destined proteins, which are mostly nucleus‐encoded. Recent investigations have focused on the postulated translocon of the second outermost membrane (periplastidal membrane, PPM). These studies identified a symbiont‐specific ERAD (endoplasmic reticulum‐associated degradation)‐like machinery (SELMA), which has been implicated in plastid pre‐protein import. Despite this recent progress, key factors for protein transport via SELMA are still unknown. As SELMA substrates presumably undergo ubiquitination, a corresponding ubiquitin ligase and an enzyme for the subsequent removal of ubiquitin need to reside in the space between the second and third membrane (periplastidal compartment, PPC). Here we characterize two proteins fulfilling these criteria. We show that ptE3P ( P. t ricornutum E3 enzyme of the P PC), the ubiquitin ligase, and ptDUP ( P. t ricornutum d e‐ u biquitinating enzyme of the P PC), the de‐ubiquitinase, localize to the PPM and PPC, respectively. In addition, we demonstrate their retained functionality by in vitro data.