MAP-1 and IAP-1, two novel AAA proteases with catalytic sites on opposite membrane surfaces in mitochondrial inner membrane of Neurospora crassa

Eukaryotic AAA proteases form a conserved family of membrane-embedded ATF-d ependent proteases but have been analyzed functionally only in the yeast Sa ccharomyces cerevisiae. Here, we have identified two novel members of this protein family in the filamentous fungus Neurospora crassa, which were term ed MAP-1 and IAP-1. Both proteins are localized to the inner membrane of mi tochondria. They are part of two similar-sized high molecular mass complexe s, but expose their catalytic sites to opposite membrane surfaces, namely, the intermembrane and the matrix space. Disruption of iap-1 by repeat-induc ed point mutation caused a slow growth phenotype at high temperature and st abilization of a misfolded inner membrane protein against degradation. IAP- 1 could partially substitute for functions of its yeast homolog Yme1, demon strating functional conservation. However, respiratory growth at 37 degrees C was not restored. Our results identify two components of the quality cont rol system. of the mitochondrial inner membrane in N. crassa and suggest th at AAA proteases with catalytic sites exposed to opposite membrane surfaces are present in mitochondria of all eukaryotic cells.