Protein unfolding is a key step in several cellular processes, including protein translocation across some membranes and protein degradation by ATP-dependent proteases. ClpAP protease and the proteasome can actively unfold proteins in a process that hydrolyzes ATP. Here we show that these proteases seem to catalyze unfolding by processively unraveling their substrates from the attachment point of the degradation signal. As a consequence, the ability of a protein to be degraded depends on its structure as well as its stability. In multidomain proteins, independently stable domains are unfolded sequentially. We show that these results can explain the limited degradation by the proteasome that occurs in the processing of the precursor of the transcription factor NF-kappaB.
The interactions between pentacene and the Si(001)-(2 × 1) surface have been investigated using Fourier transform infrared spectroscopy (FTIR), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). The pentacene molecules in the first layer react with the Si surface atoms through the CdC double bonds and via cleavage of C-H bonds. This chemisorption is accompanied by disruption of the conjugated π electron system. The disrupted interfacial layer is stable throughout deposition and evaporation of thicker pentacene films. Pentacene molecules in layers beyond the first layer adsorb molecularly and yield well-defined valence band features that are characteristic of a conjugated π electron system. Functionalization of the Si surface with a monolayer of cyclopentene inhibits dissociation of subsequently deposited pentacene molecules.
Most mitochondrial proteins are imported into mitochondria through transmembrane channels composed largely, and perhaps exclusively, of proteins. We have determined the effective internal diameter of the protein import channel in the mitochondrial outer membrane to be between 20 Å and 26 Å during translocation. The diameter of the import channel in the inner membrane is smaller than the diameter of the outer membrane import channel. These results were obtained by measuring the effect of rigid steric bulk introduced into precursor proteins on import.
Protein unfolding is a key step in the life cycle of many proteins, including certain proteins that are degraded by ATP-dependent proteases or translocated across membranes. The detailed mechanisms of these unfolding processes are not understood. Precursor proteins are unfolded and imported into mitochondria by a macromolecular machine that spans two membranes and contains at least nine different proteins. Here we examine import of a model precursor protein derived from the ribonuclease barnase and show that mitochondria unfold this protein by unraveling it from its N-terminus. Because barnase in free-solution unfolds by a different pathway, our results demonstrate that mitochondria catalyze unfolding in the way that enzymes catalyze reactions, namely by changing reaction pathways. The effectiveness of this mechanism depends on the structure of the N-terminal part of the precursor protein.
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