Polynucleotide phosphorylase (PNPase) is an exoribonuclease and poly(A) polymerase postulated to function in the cytosol and mitochondrial matrix. Prior overexpression studies resulted in PNPase localization to both the cytosol and mitochondria, concurrent with cytosolic RNA degradation and pleiotropic cellular effects, including growth inhibition and apoptosis, that may not reflect a physiologic role for endogenous PNPase. We therefore conducted a mechanistic study of PNPase biogenesis in the mitochondrion. Interestingly, PNPase is localized to the intermembrane space by a novel import pathway. PNPase has a typical N-terminal targeting sequence that is cleaved by the matrix processing peptidase when PNPase engaged the TIM23 translocon at the inner membrane. The i-AAA protease Yme1 mediated translocation of PNPase into the intermembrane space but did not degrade PNPase. In a yeast strain deleted for Yme1 and expressing PNPase, nonimported PNPase accumulated in the cytosol, confirming an in vivo role for Yme1 in PNPase maturation. PNPase localization to the mitochondrial intermembrane space suggests a unique role distinct from its highly conserved function in RNA processing in chloroplasts and bacteria. Furthermore, Yme1 has a new function in protein translocation, indicating that the intermembrane space harbors diverse pathways for protein translocation.

The TIM22 protein import pathway mediates the import of membrane proteins into the mitochondrial inner membrane and consists of two intermembrane space chaperone complexes, the Tim9-Tim10 and Tim8-Tim13 complexes. To facilitate mechanistic studies, we developed a chemical-genetic approach to identify small molecule agonists that caused lethality to a tim10-1 yeast mutant at the permissive temperature. One molecule, MitoBloCK-1, attenuated the import of the carrier proteins including the ADP/ATP and phosphate carriers, but not proteins that used the TIM23 or the Mia40/ Erv1 translocation pathways. MitoBloCK-1 impeded binding of the Tim9-Tim10 complex to the substrate during an early stage of translocation, when the substrate was crossing the outer membrane. As a probe to determine the substrate specificity of the small Tim proteins, MitoBloCK-1 impaired the import of Tim22 and Tafazzin, but not Tim23, indicating that the Tim9-Tim10 complex mediates the import of a subset of inner membrane proteins. MitoBloCK-1 also inhibited growth of mammalian cells and import of the ADP/ATP carrier, but not TIM23 substrates, confirming that MitoBloCK-1 can be used to understand mammalian mitochondrial import and dysfunction linked to inherited human disease. Our approach of screening chemical libraries for compounds causing synthetic genetic lethality to identify inhibitors of mitochondrial protein translocation in yeast validates the generation of new probes to facilitate mechanistic studies in yeast and mammalian mitochondria.chemical biology | chemical genetics T he mitochondrion has an outer (OM) and inner (IM) membrane that separates the matrix from the intermembrane space (IMS). The mitochondrion has developed an elaborate translocation system to orchestrate the import and subsequent sorting of proteins to the correct compartment (1). Proteins destined for the mitochondrion, termed precursors until they reach their correct location, utilize Translocase of the Outer Membrane (TOM) and Translocase of the Inner Membrane (TIM) complexes, TIM23 and TIM22, to cross the OM and IM, respectively. Proteins with a typical N-terminal targeting sequence use the TIM23 translocation system, whereas proteins destined for the IM use the TIM22 translocation system.Components of the TIM22 translocation system include the small Tim proteins, Tim8, Tim9, Tim10, Tim12, and Tim13, and the membrane components Tim18, Tim22, and Tim54. The small Tim proteins assemble in 70-kDa hexameric complexes (referred to as small Tim complexes) in the IMS in which three Tim9 polypeptides partner with three Tim10 polypeptides, and three Tim8 polypeptides partner with three Tim13 polypeptides. Structural studies reveal that the overall structure is similar to that of the Skp and prefoldin chaperones (2), although the sequences are not conserved. The small Tim proteins function as chaperones to maintain the hydrophobic membrane proteins in an import competent state (3 and 4). The 300-kDa insertion complex in the IM consists of a fraction of Tim9 and Tim10 with T...

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