LONP1 is the principal AAA+ unfoldase and bulk protease in the mitochondrial matrix, so its deletion causes embryonic lethality. The AAA+ unfoldase CLPX and the peptidase CLPP also act in the matrix, conspicuously during stress periods, but their substrates are poorly defined. Mammalian CLPP deletion triggers infertility, deafness, growth retardation, and cGAS-STING activated cytosolic innate immunity. CLPX mutations impair heme biosynthesis and heavy metal homeostasis. CLPP and CLPX are conserved from bacteria to human, despite their secondary role for proteolysis. Based on recent proteomic-metabolomic evidence from knockout mice and patient cells, we propose that CLPP acts on phase-separated ribonucleoprotein granules, and CLPX on multi-enzyme condensates, near the inner mitochondrial membrane, as first-aid system. Trimming within assemblies, CLPP rescues stalled processes in mitoribosomes, in mitochondrial RNA granules and nucleoids, and in D-foci-mediated degradation of toxic double-stranded mtRNA / mtDNA. Unfolding multi-enzyme condensates, CLPX maximizes PLP-dependent delta-transamination, and rescues malformed nascent peptides. Overall, their actions occur in granules with multivalent or hydrophobic interactions, separated from the aqueous phase. Thus, the role of CLPXP in the matrix is compartment-selective, like peptidases MPP at precursor import pores, m-AAA and i-AAA at either IMM face, PARL within the IMM, and OMA1/HTRA2 in the intermembrane space.

The conformational dependence of the matrix element for spin-orbit coupling and of the electronic coupling for charge separation are determined for an electron donor-acceptor system containing a pyrene acceptor and a dimethylaniline donor. Different kinetic and energetic aspects that play a role in the spin-orbit charge transfer intersystem crossing (SOCT-ISC) mechanism are discussed. This includes parameters related to initial charge separation and the charge recombination pathways using the (Semi-Classical) Marcus Theory for electron transfer. The spin-orbit coupling, which plays a significant role in charge recombination to the triplet state can be probed by (TD-)DFT, using the latter as a tool to understand and predict the SOCT-ISC mechanism. The matrix elements for spin-orbit coupling for acetone and 4-thio-thymine are used for benchmarking. (Time Dependent-) Density Functional Theory (DFT and TD-DFT) calculations are applied using the quantum chemical program Amsterdam Density Functional (ADF).

Lifespan extension was recently achieved in Caenorhabditis elegans nematodes by mitochondrial stress and mitophagy, triggered via iron depletion. Conversely in man, deficient mitophagy due to Pink1/Parkin mutations triggers iron accumulation in patient brain and limits survival. We now aimed to identify murine fibroblast factors, which adapt their mRNA expression to acute iron manipulation, relate to mitochondrial dysfunction and may influence survival. After iron depletion, expression of the plasma membrane receptor Tfrc with its activator Ireb2, the mitochondrial membrane transporter Abcb10, the heme-release factor Pgrmc1, the heme-degradation enzyme Hmox1, the heme-binding cholesterol metabolizer Cyp46a1, as well as the mitophagy regulators Pink1 and Parkin showed a negative correlation to iron levels. After iron overload, these factors did not change expression. Conversely, a positive correlation of mRNA levels with both conditions of iron availability was observed for the endosomal factors Slc11a2 and Steap2, as well as for the iron-sulfur-cluster (ISC)-containing factors Ppat, Bdh2 and Nthl1. Positive correlation only after iron depletion was observed for the iron export factor Slc40a1, mitochondrial iron transporters Slc25a28, Abcb7 and Abcb8, mitochondrial ISC-containing factors Glrx5, Nfu1, Bola1 and Abce1, cytosolic Aco1 and Tyw5, as well as nuclear Dna2, Elp3, Pold1 and Prim2. The latter are regulators of nucleotide synthesis and DNA quality control, which have known importance for growth and lifespan. The only Pink1-/- triggered transcript modulation was the reduced expression of the ISC-containing ribosomal factor Abce1. These mammalian findings support previous fly data that Pink1 influences co-translational quality control via Abce1, as well as mitophagy. Our findings provide the first systematic survey how iron dosage triggers homeostatic transcriptional regulations and elucidate how iron deprivation results in mitophagy.