Human KPNA1 blocking peptide

Satellite cells are stem cells with an essential role in skeletal muscle repair. Precise regulation of gene expression is critical for proper satellite cell quiescence, proliferation, differentiation and self-renewal. Nuclear proteins required for gene expression are dependent on the nucleocytoplasmic transport machinery to access to nucleus, however little is known about regulation of nuclear transport in satellite cells. The best characterized nuclear import pathway is classical nuclear import which depends on a classical nuclear localization signal (cNLS) in a cargo protein and the heterodimeric import receptors, karyopherin alpha (KPNA) and beta (KPNB). Multiple KPNA1 paralogs exist and can differ in importing specific cNLS proteins required for cell differentiation and function. We show that transcripts for six Kpna paralogs underwent distinct changes in mouse satellite cells during muscle regeneration accompanied by changes in cNLS proteins in nuclei. Depletion of KPNA1, the most dramatically altered KPNA, caused satellite cells in uninjured muscle to prematurely activate, proliferate and undergo apoptosis leading to satellite cell exhaustion with age. Increased proliferation of satellite cells led to enhanced muscle regeneration at early stages of regeneration. In addition, we observed impaired nuclear localization of two key KPNA1 cargo proteins: p27, a cyclin-dependent kinase inhibitor associated with cell cycle control and lymphoid enhancer factor 1, a critical cotranscription factor for beta-catenin. These results indicate that regulated nuclear import of proteins by KPNA1 is critical for satellite cell proliferation and survival and establish classical nuclear import as a novel regulatory mechanism for controlling satellite cell fate.

Inactivation of the protein complex 'mechanistic target of rapamycin complex 1' (mTORC1) can increase the nuclear content of transcriptional regulators of metabolism and apoptosis. Previous studies established that nuclear import of signal transducer and activator of transcription-alpha 1(STAT1) requires the mTORC1-associated adaptor karyopherin-alpha 1 (KPNA1) when mTORC1 activity is reduced. However, the role of other mTORC1-interacting proteins in the complex, including 'protein kinase C delta' (PKC delta), have not been well characterized. In this study, we demonstrate that PKC delta, a STAT1 kinase, contains a functional 'target of rapamycin signaling' (TOS) motif that directs its interaction with mTORC1. Depletion of KPNA1 by RNAi prevented the nuclear import of PKC delta in cells exposed to the mTORC1 inhibitor rapamycin or amino acid restriction. Mutation of the TOS motif in PKC delta led to its loss of regulation by mTORC1 or karyopherin-alpha 1, resulting in increased constitutive nuclear content. In cells expressing wild-type PKC delta, STAT1 activity and apoptosis were increased by rapamycin or interferon-beta. Those expressing the PKC delta TOS mutant exhibited increased STAT1 activity and apoptosis; further enhancement by rapamycin or interferon-beta, however, was lost. Therefore, the TOS motif in PKC delta is a novel structural mechanism by which mTORC1 prevents PKC delta and STAT1 nuclear import, and apoptosis.