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Eukaryotic Cell, February 2009, p. 134-146, Vol. 8, No. 2
1535-9778/09/$08.00+0     doi:10.1128/EC.00279-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Dual Functions for the Schizosaccharomyces pombe Inositol Kinase Ipk1 in Nuclear mRNA Export and Polarized Cell Growth{triangledown} ,{dagger}

Bhaskarjyoti Sarmah and Susan R. Wente*

Department of Cell and Developmental Biology, Vanderbilt University Medical Center, Nashville, Tennessee 37232-8240

Received 21 August 2008/ Accepted 23 November 2008

The inositol 1,3,4,5,6-pentakisphosphate (IP5) 2-kinase (Ipk1) catalyzes the production of inositol hexakisphosphate (IP6) in eukaryotic cells. Previous studies have shown that IP6 is required for efficient nuclear mRNA export in the budding yeast Saccharomyces cerevisiae. Here, we report the first functional analysis of ipk1+ in Schizosaccharomyces pombe. S. pombe Ipk1 (SpIpk1) is unique among Ipk1 orthologues in that it harbors a novel amino (N)-terminal domain with coiled-coil structural motifs similar to those of BAR (Bin-amphiphysin-Rvs) domain proteins. Mutants with ipk1+ deleted (ipk1{Delta}) had mRNA export defects as well as pleiotropic defects in polarized growth, cell morphology, endocytosis, and cell separation. The SpIpk1 catalytic carboxy-terminal domain was required to rescue these defects, and the mRNA export block was genetically linked to SpDbp5 function and, likely, IP6 production. However, the overexpression of the N-terminal domain alone also inhibited these functions in wild-type cells. This revealed a distinct noncatalytic function for the N-terminal domain. To test for connections with other inositol polyphosphates, we also analyzed whether the loss of asp1+ function, encoding an IP6 kinase downstream of Ipk1, had an effect on ipk1{Delta} cells. The asp1{Delta} mutant alone did not block mRNA export, and its cell morphology, polarized growth, and endocytosis defects were less severe than those of ipk1{Delta} cells. Moreover, ipk1{Delta} asp1{Delta} double mutants had altered inositol polyphosphate levels distinct from those of the ipk1{Delta} mutant. This suggested novel roles for asp1+ upstream of ipk1+. We propose that IP6 production is a key signaling linchpin for regulating multiple essential cellular processes.

* Corresponding author. Mailing address: Department of Cell and Developmental Biology, Vanderbilt University Medical Center, U-3209 MRBIII, Nashville, TN 37232-8240. Phone: (615) 936-3443. Fax: (615) 936-3439. E-mail: susan.wente{at}vanderbilt.edu

{triangledown} Published ahead of print on 1 December 2008.

{dagger} Supplemental material for this article may be found at http://ec.asm.org/.

Eukaryotic Cell, February 2009, p. 134-146, Vol. 8, No. 2
1535-9778/09/$08.00+0     doi:10.1128/EC.00279-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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