Tim54p connects inner membrane assembly and proteolytic pathways in the mitochondrion

Tim54p, a component of the inner membrane TIM22 complex, does not directly mediate the import of inner membrane substrates but is required for assembly/stability of the 300-kD TIM22 complex. In addition, Δtim54 yeast exhibit a petite-negative phenotype (also observed in yeast harboring mutations in the F1Fo ATPase, the ADP/ATP carrier, mitochondrial morphology components, or the i–AAA protease, Yme1p). Interestingly, other import mutants in our strain background are not petite-negative. We report that Tim54p is not involved in maintenance of mitochondrial DNA or mitochondrial morphology. Rather, Tim54p mediates assembly of an active Yme1p complex, after Yme1p is imported via the TIM23 pathway. Defective Yme1p assembly is likely the major contributing factor for the petite-negativity in strains lacking functional Tim54p. Thus, Tim54p has two independent functions: scaffolding/stability for the TIM22 membrane complex and assembly of Yme1p into a proteolytically active complex. As such, Tim54p links protein import, assembly, and turnover pathways in the mitochondrion

Mia40 Protein Serves as an Electron Sink in the Mia40-Erv1 Import Pathway

A redox-regulated import pathway consisting of Mia40 and Erv1 mediates the import of cysteine-rich proteins into the mitochondrial intermembrane space. Mia40 is the oxidoreductase that inserts two disulfide bonds into the substrate simultaneously. However, Mia40 has one redox-active cysteine pair, resulting in ambiguity about how Mia40 accepts numerous electrons during substrate oxidation. In this study, we have addressed the oxidation of Tim13 in vitro and in organello. Reductants such as glutathione and ascorbate inhibited both the oxidation of the substrate Tim13 in vitro and the import of Tim13 and Cmc1 into isolated mitochondria. In addition, a ternary complex consisting of Erv1, Mia40, and substrate, linked by disulfide bonds, was not detected in vitro. Instead, Mia40 accepted six electrons from substrates, and this fully reduced Mia40 was sensitive to protease, indicative of conformational changes in the structure. Mia40 in mitochondria from the erv1–101 mutant was also trapped in a completely reduced state, demonstrating that Mia40 can accept up to six electrons as substrates are imported. Therefore, these studies support that Mia40 functions as an electron sink to facilitate the insertion of two disulfide bonds into substrates

Role of tryptophan residues of Erv1: Trp95 and Trp183 are important for its folding and oxidase function

Erv1 is an FAD-dependent sulphydryl oxidase of the ERV/ALR sub-family, and an essential component of the mitochondrial import and assembly pathway. Erv1 contains six tryptophan residues, which are all located in the highly conserved C-terminal FAD-binding domain. Though important structural roles were predicted for the invariable Trp95, no experimental study has been reported. In this study, we investigated the structural and functional roles of individual Trp residues of Erv1. Six single Trp-to-Phe yeast mutant strains were generated and their effects on cell viability were tested at various temperatures. Then, the mutants were purified from E. coli. Their effects on folding, FAD-binding, and Erv1 activity were characterised. Our results showed that Erv1W95F has the strongest effect on the stability and function of Erv1, and followed by Erv1W183F. Erv1W95F results in a decrease of the Tm of Erv1 by 23°C, a significant loss of the oxidase activity, and thus causing cell growth defects at both 30°C and 37°C. Erv1W183F induces changes in the oligomerisation state of Erv1, along with a pronounced effect on the stability of Erv1 and its function at 37°C, whilst the other mutants had no clear effect on the function of Erv1 including the highly conserved Trp157 mutant. Finally, computational analysis indicates that Trp95 plays a key role in stabilising the isoalloxazine ring to interact with Cys133. Taken together, this study provided important insights into the molecular mechanism of how sulfhydryl oxidases use FAD in catalyzing disulfide bond formation

Linguistic Competence

The notion of linguistic competence as a cognitive system that produces knowledge not antecedently present in the mind of the subject, e.g., knowledge of grammatical relations in response to certain stimuli is an important contribution to philosophical understanding of linguistics, and of cognitive psychology in general. This notion has not been as well received as it should have been, in part because of certain false things that have been said about it. In particular, it has been said that a grammar of a language, conceived as a theory of linguistic competence, is an idealization, and that speakers know the rules of the correct grammar of their language. This paper shows that a theory of competence is not in any interesting sense an idealization, and that although a theory of linguistic competence, i.e. a grammar, aims at describing some real aspect of speakers, there is no reason to suppose that this is an aspect that would make it appropriate to say that speakers know the rules of the grammar