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Proteasomal degradation induced by DPP9-mediated processing competes with mitochondrial protein import

ABSTRACT: Plasticity of the proteome is critical to adapt to varying conditions. Control of mitochondrial protein import contributes to this plasticity. Here, we identified a pathway that regulates mitochondrial protein import by regulated N-terminal processing. We demonstrate that dipeptidyl peptidases 8/9 (DPP8/9) mediate the N-terminal processing of adenylate kinase 2 (AK2) en route to mitochondria. We show that AK2 is a substrate of the mitochondrial disulfide relay, thus lacking an N-terminal mitochondrial targeting sequence and undergoing comparatively slow import. DPP9-mediated processing of AK2 induces its rapid proteasomal degradation and prevents cytosolic accumulation of enzymatically active AK2. Besides AK2, we identify more than 100 mitochondrial proteins with putative DPP8/9 recognition sites and demonstrate that DPP8/9 influence the cellular levels of a number of these proteins. Collectively, we provide in this study a conceptual framework on how regulated cytosolic processing controls levels of mitochondrial proteins as well as their dual localization to mitochondria and other compartments.

SUBMITTER: Yannik Finger

PROVIDER: S-SCDT-EMBOJ-2019-103889 | biostudies-other |

REPOSITORIES: biostudies-other

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Project description:BackgroundThe deoxynucleotide-triphosphate (dNTP) hydrolase sterile alpha motif domain and HD domain 1 (SAMHD1) is a nuclear protein that inhibits HIV-1 infection in myeloid cells as well as quiescent CD4 T-cells, by decreasing the intracellular dNTP concentration below a level that is required for efficient reverse transcription. The Vpx proteins of the SIVSMM/HIV-2 lineage of lentiviruses bind SAMHD1 and recruit an ubiquitin ligase, leading to polyubiquitination and proteasomal degradation.ResultsHere, we have investigated the importance of nuclear localization for SAMHD1's antiviral function as well as its sensitivity to the Vpx protein of SIVMAC. Using GST pull down assays, as well as RNA silencing approaches, we show that SAMHD1 preferentially uses karyopherin α2 (KPNA2) and a classical N-terminal nuclear localization signal (14KRPR17) to enter the nucleus. Reduction of karyopherin β1 (KPNB1) or KPNA2 by RNAi also led to cytoplasmic re-distribution of SAMHD1. Using primary human monocyte-derived macrophages (MDM), a cell type in which SAMHD1 is naturally expressed to high levels, we demonstrate that nuclear localization is not required for its antiviral activity. Cytoplasmic SAMHD1 still binds to VpxMAC, is efficiently polyubiquitinated, but is not degraded. We also find that VpxMAC-induced SAMHD1 degradation was partially reversed by ubiquitin carrying the K48R or K11R substitution mutations, suggesting involvement of K48 and K11 linkages in SAMHD1 polyubiquitination. Using ubiquitin K-R mutants also revealed differences in the ubiquitin linkages between wild type and cytoplasmic forms of SAMHD1, suggesting a potential association with the resistance of cytoplasmic SAMHD1 to VpxMAC induced degradation.ConclusionsOur work extends published observations on SAMHD1 nuclear localization to a natural cell type for HIV-1 infection, identifies KPNA2/KPNB1 as cellular proteins important for SAMHD1 nuclear import, and indicates that components of the nuclear proteasomal degradation machinery are required for SAMHD1 degradation.

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Proteasomal degradation induced by DPP9-mediated processing competes with mitochondrial protein import

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