Dual Functions for the Schizosaccharomyces pombe Inositol Kinase Ipk1 in Nuclear mRNA Export and Polarized Cell Growth

Abstract

ABSTRACT The inositol 1,3,4,5,6-pentakisphosphate (IP 5 ) 2-kinase (Ipk1) catalyzes the production of inositol hexakisphosphate (IP 6 ) in eukaryotic cells. Previous studies have shown that IP 6 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 Δ) 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, IP 6 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 IP 6 kinase downstream of Ipk1, had an effect on ipk1 Δ cells. The asp1 Δ mutant alone did not block mRNA export, and its cell morphology, polarized growth, and endocytosis defects were less severe than those of ipk1 Δ cells. Moreover, ipk1 Δ asp1 Δ double mutants had altered inositol polyphosphate levels distinct from those of the ipk1 Δ mutant. This suggested novel roles for asp1 + upstream of ipk1 + . We propose that IP 6 production is a key signaling linchpin for regulating multiple essential cellular processes.