J. Neurochem. (2011) 117, 856–867. Abstract Intramembrane proteolysis is a conserved mechanism that regulates a variety of cellular processes ranging from transcription control to signaling. In mitochondria, the inner membrane rhomboid protease PARL has been implicated in the control of life span and apoptosis by a so far uncharacterized mechanism. Here, we show that PARL cleaves human Pink1, which is implicated in Parkinson’s disease, within its conserved membrane anchor. Mature Pink1 is then free to be released into the cytosol or the mitochondrial intermembrane space. Upon depolarization of the mitochondrial membrane potential, the canonical import of Pink1 and PARL‐catalyzed processing is blocked, leading to accumulation of the Pink1 precursor. As targeting of this precursor to the outer mitochondrial membrane has been shown to trigger mitophagy, we suggest that the PARL‐catalyzed removal of the Pink1 signal sequence in the canonical import pathway acts as a cellular checkpoint for mitochondrial integrity. Furthermore, we show that two Parkinson’s disease‐causing mutations decrease the processing of Pink1 by PARL, with attendant implications for pathogenesis.

Tail-anchored (TA) proteins are post-translationally targeted to and inserted into the endoplasmic reticulum (ER) membrane through their single C-terminal transmembrane domain. Membrane insertion of TA proteins in mammalian cells is mediated by the ATPase TRC40/Asna1 (Get3 in yeast) and a receptor in the ER membrane. We have identified tryptophan-rich basic protein (WRB), also known as congenital heart disease protein 5 (CHD5), as the ER membrane receptor for TRC40/Asna1. WRB shows sequence similarity to Get1, a subunit of the membrane receptor complex for yeast Get3. Using biochemical and cell imaging approaches, we demonstrate that WRB is an ER-resident membrane protein that interacts with TRC40/Asna1 and recruits it to the ER membrane. We identify the coiled-coil domain of WRB as the binding site for TRC40/Asna1 and show that a soluble form of the coiled-coil domain interferes with TRC40/Asna1-mediated membrane insertion of TA proteins. The identification of WRB as a component of the TRC (Get) pathway for membrane insertion of TA proteins raises new questions concerning the proposed roles of WRB (CHD5) in congenital heart disease, and heart and eye development.

Ribosome stalling during translation can potentially be harmful, and is surveyed by a conserved quality control pathway that targets the associated mRNA and nascent polypeptide chain (NC). In this pathway, the ribosome-associated quality control (RQC) complex promotes the ubiquitylation and degradation of NCs remaining stalled in the 60S subunit. NC stalling is recognized by the Rqc2/Tae2 RQC subunit, which also stabilizes binding of the E3 ligase, Listerin/Ltn1. Additionally, Rqc2 modifies stalled NCs with a carboxy-terminal, Ala- and Thr-containing extension—the 'CAT tail'. However, the function of CAT tails and fate of CAT tail-modified ('CATylated') NCs has remained unknown. Here we show that CATylation mediates formation of detergent-insoluble NC aggregates. CATylation and aggregation of NCs could be observed either by inactivating Ltn1 or by analyzing NCs with limited ubiquitylation potential, suggesting that inefficient targeting by Ltn1 favors the Rqc2-mediated reaction. These findings uncover a translational stalling-dependent protein aggregation mechanism, and provide evidence that proteins can become specifically marked for aggregation.DOI: http://dx.doi.org/10.7554/eLife.11794.001