Overexpression of yeast karyopherin Pse1p/Kap121p stimulates the mitochondrial import of hydrophobic proteins in vivo

Abstract

SummaryDuring evolution, cellular processes leading to the transfer of genetic information failed to send all the mitochondrial genes into the nuclear genome. Two mitochondrial genes are still exclusively located in the mitochondrial genome of all living organisms. They code for two highly hydrophobic proteins: the apocytochrome b and the subunit I of cytochrome oxidase. Assuming that the translocation machinery could not ef®ciently transport long hydrophobic fragments, we searched for multicopy suppressors of this physical blockage. We demonstrated that overexpression of Pse1p/Kap121p or Kap123p, which belong to the superfamily of karyopherin b proteins, facilitates the translocation of chimeric proteins containing several stretches of apocytochrome b fused to a reporter mitochondrial gene. The effect of PSE1/KAP121 overexpression (in which PSE1 is protein secretion enhancer 1) on mitochondrial import of the chimera is correlated with an enrichment of the corresponding transcript in cytoplasmic ribosomes associated with mitochondria. PSE1/KAP121 overexpression also improves the import of the hydrophobic protein Atm1p, an ABC transporter of the mitochondrial inner membrane. These results suggest that in vivo PSE1/ KAP121 overexpression facilitates, either directly or indirectly, the co-translational import of hydrophobic proteins into mitochondria.

mRNA localization to the mitochondrial surface allows the efficient translocation inside the organelle of a nuclear recoded ATP6 protein

“…Therefore, the steady-state levels of the ATP6 precursor and its ability to recognize the TOM complex in the outer mitochondrial membrane do not depend on the presence of the SOD2 39UTR. Notably, the relative proportions of ATP6 precursor and mature forms were analogous to the ones shown in cells for highly hydrophobic proteins en route to the mitochondria (Claros et al 1995;Corral-Debrinski et al 1999).…”

“…Mitochondrial sorting of mRNAs encoding mitochondrial proteins might likely represent a key step to ensuring the functionality of the corresponding polypeptides inside the organelle. In this case, a cotranslational phase might assist the import of the precursors (Corral-Debrinski et al 1999Fünfschilling and Rospert 1999;George et al 2002). We have demonstrated that in yeast, 47% of mRNAs encoding mitochondrial proteins are transported to the organelle surface (Sylvestre et al 2003b).…”

Nonstructural Protein 5A of Hepatitis C Virus Inhibits the Function of Karyopherin β3

“…Third, yeast PSE1 may enhance secretion of proteins (9). Fourth, yeast PSE1 may augment mitochondrial import of hydrophobic mitochondrial proteins (12). It is, however, not clear whether these effects of karyopherin ␤3, PSE1, and/or KAP123 are direct or indirect ones.…”

“…Analysis of yeast overexpressing PSE1 led to the conclusion that karyopherin ␤3 may also play a role in mitochondrial protein import (12). It is tempting to speculate that NS5A binding to karyopherin ␤3 could prevent normal mitochondrial protein import, resulting in alterations of mitochondrial functions.…”

“…Karyopherin ␤3 facilitated nuclear import of ribosomal proteins in an in vitro transportation assay system (33). Overexpression of yeast PSE1 resulted in an increase in protein secretion and stimulated mitochondrial import of hydrophobic proteins in yeast cells (9,12). In addition, the conditional loss of PSE1 in a strain lacking KAP123 resulted in a specific blockage of mRNA export from the nucleus (46).…”