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J. Biol. Chem., Vol. 282, Issue 8, 5443-5452, February 23, 2007

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The N-terminal Membrane Occupation and Recognition Nexus Domain of Arabidopsis Phosphatidylinositol Phosphate Kinase 1 Regulates Enzyme Activity^*

From the Plant Biology, North Carolina State University, Raleigh, North Carolina 27695-7649

The type I B family of phosphatidylinositol phosphate kinases (PIPKs) contain a characteristic region of Membrane Occupation and Recognition Nexus (MORN) motifs at the N terminus. These MORN motifs are not found in PIPKs from other eukaryotes. To understand the impact of the additional N-terminal domain on protein function and subcellular distribution, we expressed truncated and full-length versions of AtPIPK1, one member of this family of PIPKs, in Escherichia coli and in tobacco cells grown in suspension culture. Deletion of the N-terminal MORN domain (amino acids 1–251) of AtPIPK1 increased the specific activity of the remaining C-terminal peptide (MORN) >4-fold and eliminated activation by phosphatidic acid (PtdOH). PtdOH activation could also be eliminated by mutating Pro³⁹⁶ to Ala (P396A) in the predicted linker region between the MORN and the kinase homology domains. AtPIPK1 is product-activated and the MORN domain binds PtdIns(4,5)P₂. Adding back the MORN peptide to MORN or to the PtdOH-activated full-length protein increased activity 2-fold. Furthermore, expressing the MORN domain in vivo increased the plasma membrane PtdInsP kinase activity. When cells were exposed to hyperosmotic stress, the MORN peptide redistributed from the plasma membrane to a lower phase or endomembrane fraction. In addition, endogenous PtdInsP kinase activity increased in the endomembrane fraction of hyperosmotically stressed cells. We conclude that the MORN peptide can regulate both the function and distribution of the enzyme in a manner that is sensitive to the lipid environment.

Received for publication, December 11, 2006 , and in revised form, December 20, 2006.

^* This work was supported in part by funding from the North Carolina Agricultural Research Service and by a grant from the National Science Foundation (to W. F. B.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1 and S2.

¹ To whom correspondence should be addressed: Plant Biology Box 7649, NC State University, Raleigh NC 27695-7649. Tel.: 919-515-3496; Fax: 919-515-3436; E-mail: wendy_boss{at}ncsu.edu.

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