Unfolding-resistant translocase targeting -: A novel mechanism for outer mitochondrial membrane localization exemplified by the Bβ2 regulatory subunit of protein phosphatase 2A

Heterotrimeric serine/threonine protein phosphatase 2A (PP2A) consists of scaffolding ( A), catalytic ( C), and variable ( B, B', and B '') subunits. Variable subunits dictate subcellular localization and substrate specificity of the PP2A holoenzyme. The B beta regulatory subunit gene is mutated in spinocerebellar ataxia type 12, and one of its splice variants, B beta 2, targets PP2A to mitochondria to promote apoptosis in PC12 cells (Dagda, R. K., Zaucha, J. A., Wadzinski, B. E., and Strack, S. ( 2003) J. Biol. Chem. 278, 24976 - 24985). Here, we report that B beta 2 is localized to the outer mitochondrial membrane by a novel mechanism, combining a cryptic mitochondrial import signal with a structural arrest domain. Scanning mutagenesis demonstrates that basic and hydrophobic residues mediate mitochondrial association and the proapoptotic activity of B beta 2. When fused to green fluorescent protein, the N terminus of B beta 2 acts as a cleavable mitochondrial import signal. Surprisingly, full-length B beta 2 is not detectably cleaved and is retained at the outer mitochondrial membrane, even though it interacts with the TOM22 import receptor, as shown by luciferase complementation in intact cells. Mutations that open the C-terminal beta-propeller of B beta 2 facilitate mitochondrial import, indicating that this rigid fold acts as a stop-transfer domain by resisting the partial unfolding step prerequisite for matrix translocation. Because hybrids of prototypical import and beta-propeller domains recapitulate this behavior, we predict the existence of other similarly localized proteins and a selection against highly stable protein folds in the mitochondrial matrix. This unfolding-resistant targeting to the mitochondrial translocase is necessary but not sufficient for the proapoptotic activity of B beta 2, which also requires association with the rest of the PP2A holoenzyme.