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(33) FUNCTIONAL RELATIONSHIPS BETWEEN YFH1- AND SSQ1P: PROTEINS PARTICIPATING IN MITOCHONDRIAL IRON HOMEOSTASIS D. Pain, D.M. Gordon, M. Kogan,
S.A.B. Knight and A. Dancis
While iron is indispensable for most forms of life, excess iron can be toxic. Two mitochondrial proteins, Yfh1p and Ssq1p, play critical roles in mitochondrial iron homeostasis in yeast. Yfh1p is the yeast homolog of human frataxin, implicated in the neurodegenerative disease, Friedreich ataxia. Ssq1p is homologous to the Hsp70 chaperone Ssc1p which participates in mitochondrial protein import and folding. Both Dyfh1 and ssq1-1 mutants drastically accumulate iron in mitochondria; this prompted us to investigate the role of Ssq1p in Yfh1p biogenesis and functions. Upon import, the Yfh1p signal sequence is processed in two steps, and the second processing step is impaired in isolated ssq1-1 mitochondria. This maturation defect is specific for Yfh1p imported into ssq1-1 mitochondria and is not due to toxic effects of accumulated mitochondrial iron. Both cleavages are mediated by the matrix processing peptidase, and these sites have been precisely mapped. The first cleaved domain (domain I) is necessary and sufficient for mitochondrial targeting and import of a non-mitochondrial passenger protein. However, domain I alone cannot support efficient import of mature Yfh1p. The second cleaved domain (domain II) is required for efficient import of Yfh1p both in vitro and in vivo, performing a critical spacer and/or sorting function. Non-processing of domain II does not interfere with Yfh1p function, showing that the defective processing of Yfh1p does not cause the iron accumulation of ssq1-1 mitochondria. Yfh1p protein levels are reduced to 50% of wild-type in ssq1-1 cells and the combined deletions of SSQ1 and YFH1 are lethal. This suggests that the Ssq1p chaperone serves functions in addition to facilitating the second processing step of Yfh1p.
For further information contact...Carmen Mannella: carmen@wadsworth.org |