An intermolecular hydrogen bonded network in the PRELID-TRIAP protein family plays a role in lipid sensing.

The PRELID-TRIAP1 family of proteins is responsible for lipid transfer in mitochondria. Multiple structures have been resolved of apo and lipid substrate bound forms, allowing us to begin to piece together the molecular level details of the full lipid transfer cycle. Here, we used molecular dynamics simulations to demonstrate that the lipid binding is mediated by an extended, water-mediated hydrogen bonding network. A key mutation, R53E, was found to disrupt this network, causing lipid to be released from the complex. The X-ray crystal structure of R53E was captured in a fully closed and apo state. Lipid transfer assays and molecular simulations allow us to interpret the observed conformation in the context of the biological role. Together, our work provides further understanding of the mechanistic control of lipid transport by PRELID-TRIAP1 in mitochondria.

Disturbed intramitochondrial phosphatidic acid transport impairs cellular stress signaling.

Lipid transfer proteins of the Ups1/PRELID1 family facilitate the transport of phospholipids across the intermembrane space of mitochondria in a lipid-specific manner. Heterodimeric complexes of yeast Ups1/Mdm35 or human PRELID1/TRIAP1 shuttle phosphatidic acid (PA) mainly synthesized in the endoplasmic reticulum (ER) to the inner membrane, where it is converted to cardiolipin (CL), the signature phospholipid of mitochondria. Loss of Ups1/PRELID1 proteins impairs the accumulation of CL and broadly affects mitochondrial structure and function. Unexpectedly and unlike yeast cells lacking the CL synthase Crd1, Ups1-deficient yeast cells exhibit glycolytic growth defects, pointing to functions of Ups1-mediated PA transfer beyond CL synthesis. Here, we show that the disturbed intramitochondrial transport of PA in ups1Δ cells leads to altered unfolded protein response (UPR) and mTORC1 signaling, independent of disturbances in CL synthesis. The impaired flux of PA into mitochondria is associated with the increased synthesis of phosphatidylcholine and a reduced phosphatidylethanolamine/phosphatidylcholine ratio in the ER of ups1Δ cells which suppresses the UPR. Moreover, we observed inhibition of target of rapamycin complex 1 (TORC1) signaling in these cells. Activation of either UPR by ER protein stress or of TORC1 signaling by disruption of its negative regulator, the Seh1-associated complex inhibiting TORC1 complex, increased cytosolic protein synthesis, and restored glycolytic growth of ups1Δ cells. These results demonstrate that PA influx into mitochondria is required to preserve ER membrane homeostasis and that its disturbance is associated with impaired glycolytic growth and cellular stress signaling.

Assays for Mitophagy in Yeast.

Elimination of damaged or surplus mitochondria is crucial to maintain cellular integrity and an energy supply-demand balance. Mitophagy serves to selectively catabolize mitochondria in a manner dependent on autophagy, and contributes to mitochondrial quality and quantity control. This degradation system is highly conserved among eukaryotes including the budding yeast, Saccharomyces cerevisiae. Therefore, analyses of mitophagy using yeast have the potential to provide insights into the common mechanisms of mitophagy. Here, we introduce experimental approaches with fluorescence microscopy and western blotting to validate mitophagy in yeast. The former is useful to visualize transport of mitochondria to the vacuole, a lytic compartment, and formation of mitochondria-containing autophagosomes. The latter allows us to quantify mitochondria degradation.

Phospholipid methylation controls Atg32-mediated mitophagy and Atg8 recycling.

Degradation of mitochondria via selective autophagy, termed mitophagy, contributes to mitochondrial quality and quantity control whose defects have been implicated in oxidative phosphorylation deficiency, aberrant cell differentiation, and neurodegeneration. How mitophagy is regulated in response to cellular physiology remains obscure. Here, we show that mitophagy in yeast is linked to the phospholipid biosynthesis pathway for conversion of phosphatidylethanolamine to phosphatidylcholine by the two methyltransferases Cho2 and Opi3. Under mitophagy-inducing conditions, cells lacking Opi3 exhibit retardation of Cho2 repression that causes an anomalous increase in glutathione levels, leading to suppression of Atg32, a mitochondria-anchored protein essential for mitophagy. In addition, loss of Opi3 results in accumulation of phosphatidylmonomethylethanolamine (PMME) and, surprisingly, generation of Atg8-PMME, a mitophagy-incompetent lipid conjugate of the autophagy-related ubiquitin-like modifier. Amelioration of Atg32 expression and attenuation of Atg8-PMME conjugation markedly rescue mitophagy in opi3-null cells. We propose that proper regulation of phospholipid methylation is crucial for Atg32-mediated mitophagy.

Protein N-terminal Acetylation by the NatA Complex Is Critical for Selective Mitochondrial Degradation.

Mitophagy is an evolutionarily conserved autophagy pathway that selectively degrades mitochondria. Although it is well established that this degradation system contributes to mitochondrial quality and quantity control, mechanisms underlying mitophagy remain largely unknown. Here, we report that protein N-terminal acetyltransferase A (NatA), an enzymatic complex composed of the catalytic subunit Ard1 and the adaptor subunit Nat1, is crucial for mitophagy in yeast. NatA is associated with the ribosome via Nat1 and acetylates the second amino acid residues of nascent polypeptides. Mitophagy, but not bulk autophagy, is strongly suppressed in cells lacking Ard1, Nat1, or both proteins. In addition, loss of NatA enzymatic activity causes impairment of mitochondrial degradation, suggesting that protein N-terminal acetylation by NatA is important for mitophagy. Ard1 and Nat1 mutants exhibited defects in induction of Atg32, a protein essential for mitophagy, and formation of mitochondria-specific autophagosomes. Notably, overexpression of Atg32 partially recovered mitophagy in NatA-null cells, implying that this acetyltransferase participates in mitophagy at least in part via Atg32 induction. Together, our data implicate NatA-mediated protein modification as an early regulatory step crucial for efficient mitophagy.

PINK1/Parkin-mediated mitophagy in mammalian cells.

Mitochondria-specific autophagy (mitophagy) is a fundamental process critical for maintaining mitochondrial fitness in a myriad of cell types. Particularly, mitophagy contributes to mitochondrial quality control by selectively eliminating dysfunctional mitochondria. In mammalian cells, the Ser/Thr kinase PINK1 and the E3 ubiquitin ligase Parkin act cooperatively in sensing mitochondrial functional state and marking damaged mitochondria for disposal via the autophagy pathway. Notably, ubiquitin and deubiquitinases play vital roles in modulating Parkin activity and mitophagy efficiency. In this review, we highlight recent breakthroughs addressing the key issues of how PINK1 activates Parkin in response to mitochondrial malfunction, how Parkin localizes specifically to impaired mitochondria, and how ubiquitination and deubiquitination regulate PINK1/Parkin-mediated mitophagy.

Mitochondrial degradation during starvation is selective and temporally distinct from bulk autophagy in yeast.

Selective degradation of mitochondria is a fundamental process that depends on formation of autophagy-related double-membrane vesicles exclusive to mitochondria, and is thus termed mitophagy. In yeast, mitophagy is induced by a shift from respiration to starvation, or prolonged respiratory growth. Here we show that mitochondrial degradation in yeast also occurs selectively under starvation conditions even without respiration. Induction of mitophagy takes place much later than that of bulk autophagy, requiring Atg11 and Atg32 essential for mitophagy as well as Atg17, Atg29, and Atg31 specific for bulk autophagy. We propose that these two discrete protein complexes cooperatively activate starvation-induced mitophagy.

Dynamic regulation of myosin light chain phosphorylation by Rho-kinase.

Myosin light chain (MLC) phosphorylation plays important roles in various cellular functions such as cellular morphogenesis, motility, and smooth muscle contraction. MLC phosphorylation is determined by the balance between activities of Rho-associated kinase (Rho-kinase) and myosin phosphatase. An impaired balance between Rho-kinase and myosin phosphatase activities induces the abnormal sustained phosphorylation of MLC, which contributes to the pathogenesis of certain vascular diseases, such as vasospasm and hypertension. However, the dynamic principle of the system underlying the regulation of MLC phosphorylation remains to be clarified. Here, to elucidate this dynamic principle whereby Rho-kinase regulates MLC phosphorylation, we developed a mathematical model based on the behavior of thrombin-dependent MLC phosphorylation, which is regulated by the Rho-kinase signaling network. Through analyzing our mathematical model, we predict that MLC phosphorylation and myosin phosphatase activity exhibit bistability, and that a novel signaling pathway leading to the auto-activation of myosin phosphatase is required for the regulatory system of MLC phosphorylation. In addition, on the basis of experimental data, we propose that the auto-activation pathway of myosin phosphatase occurs in vivo. These results indicate that bistability of myosin phosphatase activity is responsible for the bistability of MLC phosphorylation, and the sustained phosphorylation of MLC is attributed to this feature of bistability.