Nucleocytoplasmic shuttling of human inositol phosphate multikinase is influenced by CK2 phosphorylation

Abstract

Human inositol phosphate multikinase (IPMK) is a multifunctional protein in cellular signal transduction, namely, a multispecific inositol phosphate kinase, phosphatidylinositol 3-kinase, and a scaffold within the mTOR-raptor complex. To fulfill these nuclear and cytoplasmic functions, intracellular targeting of IPMK needs to be regulated. We show here that IPMK, which has been considered to be a preferentially nuclear protein, is a nucleocytoplasmic shuttling protein, whose nuclear export is mediated by classical nuclear export receptor CRM1. We identified a functional nuclear export signal (NES) additionally to its previously described nuclear import signal (NLS). Furthermore, we describe a mechanism by which the activity of the IPMK-NLS is controlled. Protein kinase CK2 binds endogenous IPMK and phosphorylates it at serine 284. Interestingly, this phosphorylation can decrease nuclear localization of IPMK cell type specifically. A controlled nuclear import of IPMK may direct its actions either toward nuclear inositol phosphate (InsPx) metabolism or cytoplasmic actions on InsPx, phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P₂], as well as mTOR-raptor.

Arginine Transcriptional Response Does Not Require Inositol Phosphate Synthesis

“…A recent publication indicates that regulation of IPMK may take place by altering its nuclear/cytoplasmic localization (16). In addition, it has recently been discovered to interact with two chief nutrient regulators in mammalian cells.…”

The tumor suppressor SHIP1 colocalizes in nucleolar cavities with p53 and components of PML nuclear bodies

“…27 A minimum of 50 cells was randomly selected and examined after fixation in each experiment. For the determination of the nuclear/cytoplasmic ratio, fluorescence intensities of 3 rectangular regions of interest in both the nucleus and the cytoplasm of each cell were averaged to calculate the ratio of nuclear over cytoplasmatic fluorescence intensity (n/c ratio).…”

Efficacious inhibition of Importin α/β-mediated nuclear import of human inositol phosphate multikinase

“…Transient transfection using a liposome-based method, fixation with paraformaldehyde and DAPI staining of the cell nuclei were performed as described elsewhere [5]. Inhibitors and cellular stressors were added to the cells 24 h post transfection.…”

“…Epifluorescence microscopy and image analysis using ImageJ software (Rasband, W.S., NIH, Bethesda, Maryland, USA) were performed as previously described [5]. Briefly, digitized images of randomly selected cells expressing EYFP/EGFP fusion proteins were generated by the use of a video camera.…”

“…And indeed, IPMK is a nucleocytoplasmic shuttling protein possessing both functional NLS and NES motifs [5]. Remarkably, inactivation of the sole NLS motif of IPMK by substitution mutagenesis does not completely abrogate its nuclear import [6].…”