About 10% to 15% of the nuclear genes of eukaryotic organisms encode mitochondrial proteins. These proteins are synthesized in the cytosol and recognized by receptors on the surface of mitochondria. Translocases in the outer and inner membrane of mitochondria mediate the import and intramitochondrial sorting of these proteins; ATP and the membrane potential are used as energy sources. Chaperones and auxiliary factors assist in the folding and assembly of mitochondrial proteins into their native, three-dimensional structures. This review summarizes the present knowledge on the import and sorting of mitochondrial precursor proteins, with a special emphasis on unresolved questions and topics of current research.

Self-organized criticality 1 is one of the key concepts to describe the emergence of complexity in natural systems. The concept asserts that a system self-organizes into a critical state where system observables are distributed according to a power law. Prominent examples of self-organized critical dynamics include piling of granular media 2 , plate tectonics 3 and stick-slip motion 4 . Critical behaviour has been shown to bring about optimal computational capabilities 5 , optimal transmission 6 , storage of information 7 and sensitivity to sensory stimuli 8-10 . In neuronal systems, the existence of critical avalanches was predicted 11 and later observed experimentally 6,12,13 . However, whereas in the experiments generic critical avalanches were found, in the model of ref. 11 they only show up if the set of parameters is fine-tuned externally to a critical transition state. Here, we demonstrate analytically and numerically that by assuming (biologically more realistic) dynamical synapses 14 in a spiking neural network, the neuronal avalanches turn from an exceptional phenomenon into a typical and robust self-organized critical behaviour, if the total resources of neurotransmitter are sufficiently large.In multi-electrode recordings on slices of rat cortex and neuronal cultures 6,12 , neuronal avalanches were observed whose sizes were distributed according to a power law with an exponent of −3/2. The distribution was stable over a long period of time.Variations of the dynamical behaviour are induced by application or wash-out of neuromodulators. Qualitatively identical behaviour can be reached in models such as those in refs 11,15 by variations of a global connectivity parameter. In these models, criticality only shows up if the interactions are fixed precisely at a specific value or connectivity structure.Here, we study a model with activity-dependent depressive synapses and show that existence of several dynamical regimes can be reconciled with parameter-independent criticality. We find that synaptic depression causes the mean synaptic strengths to approach a critical value for a certain range of interaction parameters, whereas outside this range other dynamical behaviours are prevalent, see Fig. 1. We analytically derive an expression for the average coupling strengths among neurons and the average inter-spike intervals in a mean-field approach. The mean-field approximation is applicable here even in the critical state, because the quantities that are averaged do not exhibit power laws, but unimodal distributions. These mean values obey a self-consistency equation that allows us to identify the mechanism that drives the dynamics of the system towards the critical regime. Moreover, the critical regime induced by the synaptic dynamics is robust to parameter changes.Consider a network of N integrate-and-fire neurons. Each neuron is characterized by a membrane potential 0 < h i (t ) < θ. The neurons receive external inputs by a random process ξ τ ∈ {1,. . .,N} that selects a neuron ξ τ (t ) = i at a rate τ and a...

We describe here a pathway for the import of proteins into the intermembrane space (IMS) of mitochondria. Substrates of this pathway are proteins with conserved cysteine motifs, which are critical for import. After passage through the TOM channel, these proteins are covalently trapped by Mia40 via disulfide bridges. Mia40 contains cysteine residues, which are oxidized by the sulfhydryl oxidase Erv1. Depletion of Erv1 or conditions reducing Mia40 prevent protein import. We propose that Erv1 and Mia40 function as a disulfide relay system that catalyzes the import of proteins into the IMS by an oxidative folding mechanism. The existence of a disulfide exchange system in the IMS is unexpected in view of the free exchange of metabolites between IMS and cytosol via porin channels. We suggest that this process reflects the evolutionary origin of the IMS from the periplasmic space of the prokaryotic ancestors of mitochondria.

Vesicles coated with coat protein complex II (COPII) selectively transport molecules (cargo) and vesicle fusion proteins from the endoplasmic reticulum (ER) to the Golgi complex. We have investigated the role of coat proteins in cargo selection and recruitment. We isolated integral membrane and soluble cargo proteins destined for transport from the ER in complexes formed in the presence of Sar1 and Sec23/24, a subset of the COPII components, and GTP or GMP-PNP. Vesicle fusion proteins of the vSNARE family and Emp24, a member of a putative cargo carrier family, were also found in COPII complexes. The inclusion of amino-acid permease molecules into the complex depended on the presence of Shr3, a protein required for the permease to leave the ER. Resident ER proteins Sec61, BiP (Kar2) and Shr3 were not included in the complexes, indicating that the COPII components bound specifically to vesicle cargo. COPII-cargo complexes and putative cargo adaptor-cargo complexes were also isolated from COPII vesicles. Our results indicate that cargo packaging signals and soluble cargo adaptors are recognized by a recruitment complex comprising Sar1-GTP and Sec23/24.