Fidelity of organellar protein targeting

An Msp1-Protease Chimera Captures Transient AAA+ Interactions and Unveils Ost4 Mislocalization Errors

Membrane protein homeostasis (proteostasis) is essential for maintaining the integrity of eukaryotic organelles. Msp1 is a membrane anchored AAA+ (ATPase Associated with cellular Activities) protein that maintains mitochondrial proteostasis by extracting aberrant proteins from the outer mitochondrial membrane. A comprehensive understanding of the physiological roles of Msp1 has been hindered because AAA+ proteins interact with substrates transiently and common strategies to stabilize this interaction lead to undesirable mitochondrial phenotypes. To circumvent these drawbacks, we fused catalytically active Msp1 to the inactivated protease domain of the AAA+ protease Yme1. The resulting chimera sequesters substrates in the catalytically inactive degradation chamber formed by the protease domain. We performed mass spectrometry analysis with the Msp1-protease chimera and identified the signal anchored protein Ost4 as a novel Msp1 substrate. Topology experiments show that Ost4 adopts mixed orientations when mislocalized to mitochondria and that Msp1 extracts mislocalized Ost4 regardless of orientation. Together, this work develops new tools for capturing transient interactions with AAA+ proteins, identifies new Msp1 substrates, and shows a surprising error in targeting of Ost4.

An avoidance segment resolves a lethal nuclear-mitochondrial targeting conflict during ribosome assembly

The correct sorting of nascent ribosomal proteins from the cytoplasm to the nucleus or to mitochondria for ribosome production poses a logistical challenge for cellular targeting pathways. Here we report the discovery of a conserved mitochondrial avoidance segment (MAS) within the cytosolic ribosomal protein uS5 that resolves an evolutionary lethal conflict between the nuclear and mitochondrial targeting machinery. MAS removal mistargets uS5 to the mitochondrial matrix and disrupts the assembly of the cytosolic ribosome. The resulting lethality can be rescued by impairing mitochondrial import. We show that MAS triages nuclear targeting by disabling a cryptic mitochondrial targeting activity within uS5 and thereby prevents fatal capture by mitochondria. Our findings identify MAS as an essential acquisition by the primordial eukaryote that reinforced organelle targeting fidelity while developing an endosymbiotic relationship with its mitochondrial progenitor.

Analysis of mitochondrial biogenesis and protein localization by genetic screens and automated imaging

Budding yeast is a laboratory model of a simple eukaryotic cell. Its compact genome is very easy to edit. This allowed to create systematic collections (libraries) of yeast strains where every gene is either perturbed or tagged. Here we review how such collections were used to study mitochondrial biology by doing genetic screens. First, we introduce the principles of yeast genome editing and the basics of its life cycle that are useful for genetic experiments. Then we overview what yeast strain collections were created over the past years. We also describe the creation and the usage of the new generation of SWAP-Tag (SWAT) collections that allow to create custom libraries. We outline the principles of changing the genetic background of whole collections in parallel, and the basics of synthetic genetic array (SGA) approach. Then we review the discoveries that were made using different types of genetic screens focusing on general mitochondrial functions, proteome, and protein targeting pathways. The development of new collections and screening techniques will continue to bring valuable insight into the function of mitochondria and other organelles.

CG9920 is necessary for mitochondrial morphogenesis and individualization during spermatogenesis in Drosophila melanogaster

Drosophila melanogaster is an ideal model organism for investigating spermatogenesis due to its powerful genetics, conserved genes and visible morphology of germ cells during sperm production. Our previous work revealed that ocnus (ocn) knockdown resulted in male sterility, and CG9920 was identified as a significantly downregulated protein in fly abdomen after ocn knockdown, suggesting a role of CG9920 in male reproduction. In this study, we found that CG9920 was highly expressed in fly testes. CG9920 knockdown in fly testes caused male infertility with no mature sperms in seminal vesicles. Immunofluorescence staining showed that depletion of CG9920 resulted in scattered spermatid nuclear bundles, fewer elongation cones that did not migrate to the anterior region of the testis, and almost no individualization complexes. Transmission electron microscopy revealed that CG9920 knockdown severely disrupted mitochondrial morphogenesis during spermatogenesis. Notably, we found that CG9920 might not directly interact with Ocn, but rather was inhibited by STAT92E, which itself was indirectly affected by Ocn. We propose a possible novel pathway essential for spermatogenesis in D. melanogaster, whereby Ocn indirectly induces CG9920 expression, potentially counteracting its inhibition by the JAK-STAT signaling pathway.

Identification of TMEM126A as OXA1L-interacting protein reveals cotranslational quality control in mitochondria

Cellular proteostasis requires transport of polypeptides across membranes. Although defective transport processes trigger cytosolic rescue and quality control mechanisms that clear translocases and membranes from unproductive cargo, proteins that are synthesized within mitochondria are not accessible to these mechanisms. Mitochondrial-encoded proteins are inserted cotranslationally into the inner membrane by the conserved insertase OXA1L. Here, we identify TMEM126A as a OXA1L-interacting protein. TMEM126A associates with mitochondrial ribosomes and translation products. Loss of TMEM126A leads to the destabilization of mitochondrial translation products, triggering an inner membrane quality control process, in which newly synthesized proteins are degraded by the mitochondrial iAAA protease. Our data reveal that TMEM126A cooperates with OXA1L in protein insertion into the membrane. Upon loss of TMEM126A, the cargo-blocked OXA1L insertase complexes undergo proteolytic clearance by the iAAA protease machinery together with its cargo.

Mitochondria in cancer: clean windmills or stressed tinkerers?

There is now a consensus that mitochondria are important tumor drivers, sophisticated biological machines that can engender a panoply of key disease traits. How this happens, however, is still mostly elusive. The opinion presented here is that what cancer exploits are not the normal mitochondria of oxygenated and nutrient-replete tissues, but the unfit, damaged, and dysfunctional organelles generated by the hostile environment of tumor growth. These 'ghost' mitochondria survive quality control and thwart cell death to relay multiple comprehensive 'danger signals' of metabolic starvation, cellular stress, and reprogrammed gene expression. The result is a new, treacherous cellular phenotype, proliferatively quiescent but highly motile, that enables tumor cell escape from a threatening environment and colonization of distant, more favorable sites (metastasis).

Role of human HSPE1 for OPA1 processing independent of HSPD1

The human mtHSP60/HSPD1-mtHSP10/HSPE1 system prevents protein misfolding and maintains proteostasis in the mitochondrial matrix. Altered activities of this chaperonin system have been implicated in human diseases, such as cancer and neurodegeneration. However, how defects in HSPD1 and HSPE1 affect mitochondrial structure and dynamics remains elusive. In the current study, we address this fundamental question in a human cell line, HEK293T. We found that the depletion of HSPD1 or HSPE1 results in fragmentation of mitochondria, suggesting a decrease in mitochondrial fusion. Supporting this notion, HSPE1 depletion led to proteolytic inactivation of OPA1, a dynamin-related GTPase that fuses the mitochondrial membrane. This OPA1 inactivation was mediated by a stress-activated metalloprotease, OMA1. In contrast, HSPD1 depletion did not induce OMA1 activation or OPA1 cleavage. These data suggest that HSPE1 controls mitochondrial morphology through a mechanism separate from its chaperonin activity.

Role of Hsp70 in Post-Translational Protein Targeting: Tail-Anchored Membrane Proteins and Beyond

The Hsp70 family of molecular chaperones acts as a central 'hub' in the cell that interacts with numerous newly synthesized proteins to assist in their biogenesis. Apart from its central and well-established role in facilitating protein folding, Hsp70s also act as key decision points in the cellular chaperone network that direct client proteins to distinct biogenesis and quality control pathways. In this paper, we review accumulating data that illustrate a new branch in the Hsp70 network: the post-translational targeting of nascent membrane and organellar proteins to diverse cellular organelles. Work in multiple pathways suggests that Hsp70, via its ability to interact with components of protein targeting and translocation machineries, can initiate elaborate substrate relays in a sophisticated cascade of chaperones, cochaperones, and receptor proteins, and thus provide a mechanism to safeguard and deliver nascent membrane proteins to the correct cellular membrane. We discuss the mechanistic principles gleaned from better-studied Hsp70-dependent targeting pathways and outline the observations and outstanding questions in less well-studied systems.

Defective Thyroglobulin: Cell Biology of Disease

The primary functional units of the thyroid gland are follicles of various sizes comprised of a monolayer of epithelial cells (thyrocytes) surrounding an apical extracellular cavity known as the follicle lumen. In the normal thyroid gland, the follicle lumen is filled with secreted protein (referred to as colloid), comprised nearly exclusively of thyroglobulin with a half-life ranging from days to weeks. At the cellular boundary of the follicle lumen, secreted thyroglobulin becomes iodinated, resulting from the coordinated activities of enzymes localized to the thyrocyte apical plasma membrane. Thyroglobulin appearance in evolution is essentially synchronous with the appearance of the follicular architecture of the vertebrate thyroid gland. Thyroglobulin is the most highly expressed thyroid gene and represents the most abundantly expressed thyroid protein. Wildtype thyroglobulin protein is a large and complex glycoprotein that folds in the endoplasmic reticulum, leading to homodimerization and export via the classical secretory pathway to the follicle lumen. However, of the hundreds of human thyroglobulin genetic variants, most exhibit increased susceptibility to misfolding with defective export from the endoplasmic reticulum, triggering hypothyroidism as well as thyroidal endoplasmic reticulum stress. The human disease of hypothyroidism with defective thyroglobulin (either homozygous, or compound heterozygous) can be experimentally modeled in thyrocyte cell culture, or in whole animals, such as mice that are readily amenable to genetic manipulation. From a combination of approaches, it can be demonstrated that in the setting of thyroglobulin misfolding, thyrocytes under chronic continuous ER stress exhibit increased susceptibility to cell death, with interesting cell biological and pathophysiological consequences.