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My research interests center around an inositol kinase which can also function as a protein kinase and as an isomerase. This enzyme (5/6-kinase) converts inositol 1,3,4-trisphosphate to one of two inositol tetrakisphosphate isomers, either inositol 1,3,4,6-tetrakisphosphate or inositol 1,3,4,5-tetrakisphosphate. As an isomerase, it can interconvert these two IP4 isomers. This enzyme acts at a branch point in inositol metabolism, and is the rate-limiting step for the synthesis of higher phosphorylated forms of inositol, including IP6. IP6 has been implicated in a variety of cellular processes, including mRNA export from the nucleus, binding to the clathrin assembly proteins AP2 and AP180, inhibition of clathrin cage assembly in vitro, inhibition of serine and threonine protein kinases, regulation of nonhomologous DNA end joining of double strand breaks, endocytosis, adenosine deaminase function, and nonvisual arrestin oligomerization.

Unlike protein kinases and other small molecule kinases (e.g. sugar kinases), there are no canonical kinase domains, including a recognizable ATP-binding domain, in 5/6-kinase. The enzyme is highly conserved throughout evolution, and we have exploited this by mutating residues which are absolutely conserved. Each mutant has been characterized with regard to its enzymatic activity. Together with the recently solved crystal structure of the Entamoeba histolytica 5/6-kinase, we have been able to define the active site of this enzyme. We are currently assessing the protein kinase activity of these mutants to confirm that the protein kinase activity is inherent to 5/6-kinase.

Most recently we have analyzed the expression pattern of 5/6-kinase during mouse development, using in situ hybridization and Xgal staining of transgenic mice harboring a β-galactosidase gene trap cassette in the 5/6-kinase gene. The enzyme is strongly expressed in the neural tube early in development and throughout the central and peripheral nervous systems at later developmental stages, as well as in arterial smooth muscle cells and in adipose tissue, particularly brown fat. In addition, in vitro differentiation of 3T3L1 cells into adipocytes was associated with upregulation of 5/6-kinase expression.

Overall structure of 5/6-kinase

Active site of human 5/6-kinase with Ins(1,3,4)P3 and Mg2+ ions

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Updated: April 8, 2008