Phosphorylation of the outer membrane mitochondrial protein OM64 influences protein import into mitochondria.

Mitochondrial localized proteins are mostly synthesized in the cytosol and translocated across the outer mitochondrial membrane via the translocase of the outer membrane (TOM) complex. Although the channel protein is conserved among eukaryotes, the receptor proteins are more divergent and show features specific to the plant lineage. OM64, which is a paralogue of the chloroplast docking protein Toc64, is unique to plants. However, due to the presence of a cytosolic exposed TPR domain it might functionally replace yeast/mammalian Tom70, which is not found in plant mitochondria, by interacting with the C-terminal (M)EEVD motif of the heat shock proteins Hsp90 and Hsp70. In this study, we show that OM64 is phosphorylated within its TPR domain. Using isothermal titration calorimetry it could be demonstrated that phosphorylation reduces the binding affinity of OM64 to Hsp90. Moreover, in vivo expression of genes encoding different OM64 variants in planta revealed that phosphorylation of OM64 impairs the import efficiency of the mitochondrial preprotein pFAD, a subunits of the mitochondrial ATP synthase. In summary, our data provide significant insight into the fine-tuning mechanisms of mitochondrial protein import mediated by phosphorylation of the cytosolic exposed receptor protein OM64.

In Vivo Radiolabeling of Arabidopsis Chloroplast Proteins and Separation of Thylakoid Membrane Complexes by Blue Native PAGE.

The investigation of membrane protein complex assembly and degradation is essential to understand cellular protein dynamics. Blue native PAGE provides a powerful tool to analyze the composition and formation of protein complexes. Combined with in vivo radiolabeling, the synthesis and decay of protein complexes can be monitored on a timescale ranging from minutes to several hours. Here, we describe a protocol to analyze thylakoid membrane complexes starting either with (35)S-methionine labeling of intact Arabidopsis leaves to investigate protein complex dynamics or with unlabeled leaf material to monitor steady-state complex composition.

Phosphomimicking within the transit peptide of pHCF136 leads to reduced photosystem II accumulation in vivo.

Most chloroplast resident proteins are equipped with N-terminal transit peptides to ensure targeting from the cytosol to the organelle. Import rates can be modulated by phosphorylation and 14-3-3 binding within the transit peptides. Using the phosphorylatable preprotein pHCF136, a photosystem II assembly factor, we investigated the function of preprotein phosphorylation in vivo by complementing the seedling lethal hcf136 mutant. HCF136 constructs containing mutations within the 14-3-3 binding site were generated, either abolishing or mimicking phosphorylation. Interestingly, phosphomimicking reduced the import rate and the hcf136 phenotype could only be partially rescued, as shown by hampered photosystem II complex accumulation, which was most prominently observed in cotyledons.

SK channel activation modulates mitochondrial respiration and attenuates neuronal HT-22 cell damage induced by H2O2.

Previous studies established an essential role for small conductance calcium-activated potassium (SK) channels in neuronal cell death pathways induced by glutamate excitotoxicity in cortical neurons in vitro and after cerebral ischemia in vivo. In addition to the intracellular calcium deregulation, glutamate-induced cell death also involves mechanisms of oxidative stress and mitochondrial dysfunction. Therefore, we sought to investigate whether SK channel activation might also affect mechanisms of intrinsic death pathways induced by reactive oxygen species (ROS) such as hydrogen peroxide (H2O2). Exposure of immortalized hippocampal HT-22 cells to H2O2 imposed activation of a cascade of intracellular toxic events resulting in intracellular ROS production, mitochondrial loss of function, and ultimately cell death. Using a pharmacological approach to activate SK channels with CyPPA, we demonstrated a reduction of H2O2-mediated intracellular ROS production and cell death. Interestingly, CyPPA mediated neuroprotection in conditions of extracellular calcium and/or pyruvate depletion, pointing to a neuroprotective role of mitochondrial SK channels. Moreover, CyPPA partially inhibited H2O2-induced mitochondrial superoxide production, but did not prevent mitochondrial membrane depolarization. CyPPA treatment resulted in slight ATP depletion and a reduction of mitochondrial respiration/oxygen consumption. These findings postulate that SK channels mediate a protective effect by preventing neuronal death from subsequent oxidative stress through an adaptive metabolic response at the level of mitochondria. Therefore, SK channel activation may serve as a therapeutic target, where mitochondrial dysfunction and related mechanisms of oxidative stress contribute to progressive degeneration and death of neurons.