N-terminal lysines are essential for protein translocation via a modified ERAD system in complex plastids

Nuclear-encoded pre-proteins being imported into complex plastids of red algal origin have to cross up to five membranes. Thereby, transport across the second outermost or periplastidal membrane (PPM) is facilitated by SELMA (symbiont-specific ERAD-like machinery), an endoplasmic reticulum-associated degradation (ERAD)-derived machinery. Core components of SELMA are enzymes involved in ubiquitination (E1-E3), a Cdc48 ATPase complex and Derlin proteins. These components are present in all investigated organisms with four membrane-bound complex plastids of red algal origin, suggesting a ubiquitin-dependent translocation process of substrates mechanistically similar to the process of retro-translocation in ERAD. Even if, according to the current model, translocation via SELMA does not end up in the classical poly-ubiquitination, transient mono-/oligo-ubiquitination of pre-proteins might be required for the mechanism of translocation. We investigated the import mechanism of SELMA and were able to show that protein transport across the PPM depends on lysines in the N-terminal but not in the C-terminal part of pre-proteins. These lysines are predicted to be targets of ubiquitination during the translocation process. As proteins lacking the N-terminal lysines get stuck in the PPM, a 'frozen intermediate' of the translocation process could be envisioned and initially characterized.