Project description:The guided entry of tail-anchored proteins (GET) pathway assists in the proper delivery of tail-anchored (TA) proteins to the ER. Here we uncover how the yeast GET pathway components Get4/5 mediate capture of TA proteins by Sgt2, which interacts with TA sequences and hands them over to the targeting component Get3. Get4/5 binds directly and with high affinity to ribosomes, positions Sgt2 close to the ribosomal tunnel exit, and facilitates the capture of TA proteins by Sgt2. The contact sites of Get4/5 on the ribosome overlap with those of SRP, the factor mediating cotranslational ER-targeting of proteins containing internal TM domains. Exposure of nascent, internal TM domains at the tunnel exit induces high-affinity ribosome-binding of SRP, which in turn prevents ribosome-binding of Get4/5. In this way, the position of TM domains within nascent ER-targeted proteins mediates partitioning into either the GET or SRP pathway directly at the ribosomal tunnel exit.
Project description:The guided entry of tail-anchored proteins (GET) pathway assists in the posttranslational delivery of tail-anchored proteins, containing a single C-terminal transmembrane domain, to the ER. Here we uncover how the yeast GET pathway component Get4/5 facilitates capture of tail-anchored proteins by Sgt2, which interacts with tail-anchors and hands them over to the targeting component Get3. Get4/5 binds directly and with high affinity to ribosomes, positions Sgt2 close to the ribosomal tunnel exit, and facilitates the capture of tail-anchored proteins by Sgt2. The contact sites of Get4/5 on the ribosome overlap with those of SRP, the factor mediating cotranslational ER-targeting. Exposure of internal transmembrane domains at the tunnel exit induces high-affinity ribosome binding of SRP, which in turn prevents ribosome binding of Get4/5. In this way, the position of a transmembrane domain within nascent ER-targeted proteins mediates partitioning into either the GET or SRP pathway directly at the ribosomal tunnel exit.
Project description:Hundreds of proteins are inserted post-translationally into the endoplasmic reticulum (ER) membrane by a single carboxy-terminal transmembrane domain (TMD). During targeting through the cytosol, the hydrophobic TMD of these tail-anchored (TA) proteins requires constant chaperoning to prevent aggregation or inappropriate interactions. A central component of this targeting system is TRC40, a conserved cytosolic factor that recognizes the TMD of TA proteins and delivers them to the ER for insertion. The mechanism that permits TRC40 to find and capture its TA protein cargos effectively in a highly crowded cytosol is unknown. Here we identify a conserved three-protein complex composed of Bat3, TRC35 and Ubl4A that facilitates TA protein capture by TRC40. This Bat3 complex is recruited to ribosomes synthesizing membrane proteins, interacts with the TMDs of newly released TA proteins, and transfers them to TRC40 for targeting. Depletion of the Bat3 complex allows non-TRC40 factors to compete for TA proteins, explaining their mislocalization in the analogous yeast deletion strains. Thus, the Bat3 complex acts as a TMD-selective chaperone that effectively channels TA proteins to the TRC40 insertion pathway.
Project description:Tail-anchored (TA) proteins are transmembrane proteins with a single C-terminal transmembrane domain, which functions as both their subcellular targeting signal and membrane anchor. We show that knockout of TRC40 in cultured human cells has a relatively minor effect on endogenous TA proteins, despite their apparent reliance on this pathway in vitro These findings support recent evidence that the canonical TRC40 pathway is not essential for TA protein biogenesis in vivo We therefore investigated the possibility that other ER-targeting routes can complement the TRC40 pathway and identified roles for both the SRP pathway and the recently described mammalian SND pathway in TA protein biogenesis. We conclude that, although TRC40 normally plays an important role in TA protein biogenesis, it is not essential, and speculate that alternative pathways for TA protein biogenesis, including those identified in this study, contribute to the redundancy of the TRC40 pathway.
Project description:Enhanced ascorbate peroxidase 2 (APEX2) can be used as a genetic tag that produces short-lived yet highly reactive biotin species, allowing the modification of proteins that interact with or are in very close proximity to the tagged protein, ideally within a confined compartment. Biotinylated proteins can be isolated using immobilized streptavidin and analyzed by mass spectrometry. Here we used rapamycin-dependent targeting of APEX2 to tail-anchored proteins of the endoplasmic reticulum and of the inner nuclear membrane (INM). In combination with stable isotope labeling with amino acids in cell culture (SILAC), our novel approach (rapamycin- and APEX-dependent identification of proteins by SILAC, or RAPIDS) allowed the identification of proteins that are in close proximity to the prototypic INM-protein emerin and the vesicle-trafficking protein VAPB. In addition to well-known interaction partners, several new potential binding partners were identified. In RAPIDS, the comparison of plus/minus-rapamycin conditions allows a clear distinction between specific and non-specific hits.
Project description:Misfolded or damaged proteins are typically targeted for destruction by proteasome-mediated degradation, but the mammalian ubiquitin machinery involved is incompletely understood. Here, using forward genetic screens in human cells, we find that the proteasome-mediated degradation of the soluble misfolded reporter, mCherry-CL1, involves two ER-resident E3 ligases, MARCH6 and TRC8. mCherry-CL1 degradation is routed via the ER membrane and dependent on the hydrophobicity of the substrate, with complete stabilisation only observed in double knockout MARCH6/TRC8 cells. To identify a more physiological correlate, we used quantitative mass spectrometry and found that TRC8 and MARCH6 depletion altered the turnover of the tail-anchored protein heme oxygenase-1 (HO-1). These E3 ligases associate with the intramembrane cleaving signal peptide peptidase (SPP) and facilitate the degradation of HO-1 following intramembrane proteolysis. Our results highlight how ER-resident ligases may target the same substrates, but work independently of each other, to optimise the protein quality control of selected soluble and tail-anchored proteins.
Project description:Ribosome hibernation is a commonly used strategy that protects ribosomes under unfavorable conditions and regulates developmental processes. Multiple ribosome-hibernation factors have been identified in all domains of life, but due to their structural diversity and the lack of a common mechanism by which they inactivate ribosomes, it is currently unknown how many different hibernation factors exist. Here, we demonstrate that the YqjD/ElaB/YgaM protein family constitute an abundant class of ribosome-hibernating proteins and represent the first membrane-bound hibernating factors identified in bacteria. Our data demonstrate that they interact with 50S ribosomal subunit and show a concentration-dependent inhibition of protein synthesis. By combining in vivo cross-linking with mass spectrometry, we show that they bind via their N-termini to proteins that surround the ribosomal tunnel exit and even penetrate into the ribosomal tunnel. Thus, YqjD/ElaB/YgaM inhibit translation by blocking the ribosomal tunnel and thus mimic the activity of antimicrobial peptides and macrolide antibiotics.
Project description:The loss of the tail is among the notable anatomical changes to have occurred along the evolutionary lineage leading to humans and to the “anthropomorphous apes”, with a hypothesized role in contributing to human bipedalism. Yet, the genetic mechanism that facilitated tail-loss evolution in hominoids remains unknown. Here, we present evidence that an individual insertion of an Alu element in the genome of the hominoid ancestor may have contributed to tail-loss evolution. We demonstrate that this Alu element – inserted into an intron of the TBXT gene – pairs with a neighboring ancestral Alu element encoded in the reverse genomic orientation and leads to a hominoid-specific alternative splicing event. To study the effect of this splicing event, we generated multiple mouse models that express both full-length and exon-skipped isoforms of mouse Tbxt, mimicking the expression pattern of its hominoid ortholog TBXT. We found that mice expressing both Tbxt isoforms can exhibit a complete absence of the tail or a shortened tail, depending on the relative abundance of Tbxt isoforms expressed at the embryonic tail bud, supporting the notion that the exon-skipped transcript is sufficient to induce a tail-loss phenotype. We further noted that mice expressing the exon-skipped Tbxt isoform may develop neural tube defects, a condition that affects ~1/1,000 neonates in human. Thus tail-loss evolution may have been associated with an adaptive cost of the potential for neural tube defects, which continue to affect human health today.