Inositol polyphosphate multikinase (IPMK) in transcriptional regulation and nuclear inositide metabolism

Abstract

Inositol polyphosphate multikinase (IPMK, ipk2, Arg(82), ArgRIII) is an inositide kinase with unusually flexible substrate specificity and the capacity to partake in many functional protein-protein interactions (PPIs). By merging these two activities, IPMK is able to execute gene regulatory functions that are very unique and only now beginning to be recognized. In this short review, we present a brief history of IPMK, describe the structural biology of the enzyme and highlight a few recent discoveries that have shed more light on the role IPMK plays in inositide metabolism, nuclear signalling and transcriptional regulation.

Keywords: ArgR; IPMK; gene expression; inositide; ipk2; multikinase; transcription.

Figure 1. Crystal structure of yeast IPMK reveals positions of amino acids involved in IPMK substrate specificity and PPIs

(A) Ribbon cartoon of the 2.0 Å crystal structure of S. cerevisiae IPMK [43] (PDB: 2IF8), one of only two IPMK structures solved to date, the other being the A. thaliana plant IPMK [30] (result not shown, PDB: 4FRF). The helix in the lower right labelled as ‘IP loop’ is the inositol binding helix, instilling much of the substrate specificity to the enzyme. ADP indicated as sticks. α9, α5 and β6 elements have been suggested to be a PPI surface. (B) Tyr¹⁷⁴ of the mouse IPMK is phosphorylated in response to glucose and mediates IPMK interaction with AMPK, through an unknown mechanism. Tyr¹⁷⁴ is conserved in human IPMK (Tyr¹⁹¹) and the depicted yeast IPMK (Tyr²⁰²), as indicated. (C) As in (B), rotated 90°.

Figure 2. Co-crystal structures of SF-1/PIP₂ and SF-1/PIP₃ demonstrate high solvent accessibility of the inositide headgroups

(A) Ribbon cartoon of the 2.8 Å co-crystal structure of the ligand-binding domain (LBD) of the nuclear receptor SF-1 bound to PIP₂ (PDB: 4QK4), PIP₂ indicated as sticks, revealing the solvent accessibility of the PIP₂ headgroup. (B) Ribbon carton of the 2.4 Å crystal structure of the SF-1 LBD bound to PIP₃ (PDB: 4QJR), PIP₃ indicated as sticks. In both structures, the transcriptional co-activator peptide has been removed for clarity.

Figure 3. Enzyme kinetic parameters of human IPMK activity on PIP₂ in SF-1 or micelles

(A) Kinetic parameters of human IPMK activity on PIP₂ bound by the nuclear receptor SF-1 [35]. (B) Kinetic parameters of human IPMK activity on PIP₂ in micelles [35]. Although SF-1 slows the enzyme down, SF-1 improves the apparent K_M, consistent with a PPI between IPMK and SF-1 facilitating IPMK phosphorylation of PIP₂ bound by SF-1.

Figure 4. IPMK has many cell physiological roles

IPMK has well-established roles in inositol phosphate metabolism, in nuclear signalling through protein phosphoinositide complexes such as SF-1/PIP₂, as a transcriptional co-regulator and in the cytoplasm as an adapter protein mediating several metabolic responses. The central role of IPMK in such a broad array of cellular processes highlights its importance to all eukaryotic cells.