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© The Rockefeller University Press, 0021-9525/1998//1575 $5.00
The Journal of Cell Biology, Volume 141, Number 7, , 1998 1575-1587

Articles

A Role for NIMA in the Nuclear Localization of Cyclin B in Aspergillus nidulans



L. Wu*, S.A. Osmani*, and P.M. Mirabito{ddagger}

* Henry Hood Research Program, Weis Center for Research, Pennsylvania State University College of Medicine, Danville, Pennsylvania 17822-2617; and {ddagger} Molecular and Cellular Biology Section, School of Biological Sciences, University of Kentucky, Lexington, Kentucky 40506-0225

NIMA promotes entry into mitosis in late G2 by some mechanism that is after activation of the Aspergillus nidulans G2 cyclin-dependent kinase, NIMXCDC2/NIMECyclin B. Here we present two independent lines of evidence which indicate that this mechanism involves control of NIMXCDC2/NIMECyclin B localization. First, we found that NIMECyclin B localized to the nucleus and the nucleus-associated organelle, the spindle pole body, in a NIMA-dependent manner. Analysis of cells from asynchronous cultures, synchronous cultures, and cultures arrested in S or G2 showed that NIMECyclin B was predominantly nuclear during interphase, with maximal nuclear accumulation in late G2. NIMXCDC2 colocalized with NIMECyclin B in G2 cells. Although inactivation of NIMA using either the nimA1 or nimA5 temperature-sensitive mutations blocked cells in G2, NIMXCDC2/NIMECyclin B localization was predominantly cytoplasmic rather than nuclear. Second, we found that nimA interacts genetically with sonA, which is a homologue of the yeast nucleocytoplasmic transporter GLE2/RAE1. Mutations in sonA were identified as allele-specific suppressors of nimA1. The sonA1 suppressor alleviated the nuclear division and NIMECyclin B localization defects of nimA1 cells without markedly increasing NIMXCDC2 or NIMA kinase activity. These results indicate that NIMA promotes the nuclear localization of the NIMXCDC2/ NIMECyclin B complex, by a process involving SONA. This mechanism may be involved in coordinating the functions of NIMXCDC2 and NIMA in the regulation of mitosis.

Abbreviations used in this paper: CCD, charge-coupled device; cs, cold sensitive; DAPI, 4',6-diamidino-2-phenylindole; HA, hemagglutinin; ORF, open reading frame; SPB, spindle pole body; ts, temperature sensitive.

P.M. Mirabito thanks D.A. Harrison (University of Kentucky, Lexington, KY) for use of his image station and for his skill and patience in helping with imaging and figure preparation. The authors also thank members of the Mirabito and Osmani laboratories, particularly S. Venkatram, for helpful discussions throughout this work and for critical reading of the manuscript.

Address all correspondence to P.M. Mirabito, Molecular and Cellular Biology Section, School of Biological Sciences, University of Kentucky, Lexington, KY 40506-0225. Tel.: (606) 257-7642. Fax: (606) 257-1717. E-mail: pmmira000{at}pop.uky.edu


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