Nuclear-localized Calcineurin Homologous Protein CHP1 Interacts with Upstream Binding Factor and Inhibits Ribosomal RNA Synthesis

Abstract

Calcineurin homologous protein 1 (CHP1) is a widely expressed, 22-kDa myristoylated EF-hand Ca 2؉ -binding protein that shares a high degree of similarity with the regulatory B subunit of calcineurin (65%) and with calmodulin (59%). CHP1 localizes to the plasma membrane, the Golgi apparatus, and the nucleus and functions to regulate trafficking of early secretory vesicles, activation of T cells, and expression and transport of the Na-H exchanger NHE1. Although CHP1 contains nuclear export signals, whether its nuclear and cytoplasmic localization is regulated and has distinct functions remain unknown. We show that CHP1 is predominantly in the nucleus in quiescent fibrobasts, is translocated to cytoplasmic compartments with growth medium, and that translocation is inhibited by mutations in the nuclear export motifs. In a screen for proteins co-precipitating with CHP1 in quiescent cells we identified the upstream binding factor UBF, a DNA-binding protein and component of the RNA polymerase I complex regulating RNA synthesis. The CHP1-UBF interaction is restricted to the nucleus and inhibited by Ca 2؉ . Nuclear retention of CHP1 attenuates the abundance of UBF in the nucleolus and inhibits RNA synthesis when quiescent cells are transferred to growth medium. These data show UBF as a newly identified CHP1-binding protein and regulation of RNA synthesis as a newly identified function for nuclear-localized CHP1, which is distinct from CHP1 functions in the cytosol.A protein that localizes in multiple subcellular compartments often has spatially distinct functions. One example is the calcineurin B homologous protein CHP1, 4 an N-myristoylated EF-hand Ca 2ϩ -binding protein sharing ϳ40% identity with the regulatory B subunit of the protein phosphatase calcineurin and ϳ30% identity with calmodulin. CHP1, also known as p22, is evolutionarily conserved in human, rat, and Caenorhabditis elegans, is ubiquitously expressed in mammalian tissues, and localizes at the plasma membrane and in vesicular, cytosolic, and nuclear compartments. We previously identified CHP1 in a screen for proteins interacting with the C-terminal cytoplasmic domain of the plasma membrane Na-H exchanger NHE1 (1). Subsequent studies found that CHP1 is a cofactor essential for NHE1 activity (2-4) and also interacts with the Na-H exchanger isoform NHE3 (5). In vesicular compartments, CHP1 associates with membranes of the early secretory pathway (6) and is required for membrane traffic in a cell-free assay (7). In the cytoplasm, CHP1 complexes with microtubules (8) to facilitate microtubule-membrane interactions (9). Microtubule association can be mediated by direct binding of CHP1 to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (6), which associates with the microtubule cytoskeleton within the early secretory pathway, or to the kinesin-related motor KIF1〉␤2 (10), which mediates the transport of synaptic vesicles in neurons. In the cytosolic compartment CHP1 also directly binds the calcineurin catalytic A subunit and inhibits calcineurin phosphatase activ...

Pre-45s rRNA promotes colon cancer and is associated with poor survival of CRC patients

“… 21 On the other hand, the interaction between calcineurin homologous protein 1 and UBF can be abolished by the presence of Ca 2+ , thereby promoting rRNA transcription. 20 , 22 These findings collectively suggested that the effect of CaMKII-S6K-UBF on the regulation of pre-45s rRNA transcription in CRC is potentially mediated by RNA polymerase I complex.…”

“…It was reported that inhibition of Ca 2+ by an N-myristoylated EF-hand Ca 2+ -binding protein, calcineurin homologous protein 1, suppresses rRNA transcription by interacting with UBF, which is a component of the RNA polymerase I complex. 20 Inhibition of RNA polymerase I promotes the cancer-specific activation of p53 and subsequent suppression of tumor growth. 21 On the other hand, the interaction between calcineurin homologous protein 1 and UBF can be abolished by the presence of Ca 2+ , thereby promoting rRNA transcription.…”

Localization and activity of the calcineurin catalytic and regulatory subunit complex at the septum is essential for hyphal elongation and proper septation in Aspergillus fumigatus

“…Although it is well established that the dephosphorylation of Crz1p by calcineurin leads to its nuclear translocation (Stathopoulos‐Gerontides et al ., 1999), there are no reports of the nuclear localization of calcineurin B in the yeasts or filamentous fungi. Nuclear localization of a calcineurin B homologous protein (CHP1) has been reported (Nagita et al ., 2003; Jiménez‐Vidal et al ., 2010). We could not identify any potential NLS sequence in the A. fumigatus CnaB and it is possible that CnaB may translocate into the nucleus with CnaA and CrzA.…”

Calcineurin homologous protein: a multifunctional Ca2+-binding protein family

“…In the nucleus, CHP1 also associates with the upstream binding factor (UBF) and inhibits ribosomal RNA synthesis, thus indicating that nuclear-localized CHP1 is regulated and has a distinct function compared with CHP1 in other cellular compartments (57). UBFs are transcription factors that bind ribosomal DNA and form a preinitiation complex with RNA polymerase I to increase transcribed ribosomal DNA.…”