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Molecular and Cellular Biology, November 2006, p. 8488-8497, Vol. 26, No. 22
0270-7306/06/$08.00+0     doi:10.1128/MCB.01006-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

A New Function in Translocation for the Mitochondrial i-AAA Protease Yme1: Import of Polynucleotide Phosphorylase into the Intermembrane Space{triangledown} ,{dagger}

Robert N. Rainey,1 Jenny D. Glavin,1 Hsiao-Wen Chen,2 Samuel W. French,2 Michael A. Teitell,2,3,4,5 and Carla M. Koehler1,3,4*

Department of Chemistry and Biochemistry,1 Department of Pathology and Laboratory Medicine,2 Molecular Biology Institute,3 Jonsson Comprehensive Cancer Center,4 California NanoSystems Institute and the Institute for Stem Cell Biology and Medicine, UCLA, Los Angeles, California 900955

Received 5 June 2006/ Returned for modification 27 June 2006/ Accepted 30 August 2006

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.

* Corresponding author. Mailing address: Department of Chemistry and Biochemistry, Box 951569, UCLA, Los Angeles, CA 90095-1569. Phone: (310) 794-4834. Fax: (310) 206-4038. E-mail: koehler{at}chem.ucla.edu.

{triangledown} Published ahead of print on 11 September 2006.

{dagger} Supplemental material for this article may be found at http://mcb.asm.org/.

Molecular and Cellular Biology, November 2006, p. 8488-8497, Vol. 26, No. 22
0270-7306/06/$08.00+0     doi:10.1128/MCB.01006-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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