KapI, a non-essential member of the Pse1p/Imp5 karyopherin family, controls colonial and asexual development in Aspergillus nidulans

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Asexual development in the filamentous fungus Aspergillus nidulans is governed by the timely expression and cellular localization of multiple transcription factors. Hence, factors mediating import and export across the nuclear pore complexes (karyopherins) are expected to play a key role in coordinating the developmental programme. Here we characterize KapI, a putative homologue of the Saccharomyces cerevisiae Kap121/Pse1p karyopherin. KapI is a non-essential importin-beta-like protein located in the nucleus during vegetative growth and conidiophore development. The DeltakapI phenotype is aconidial with many aerial hyphae. This phenotype can be suppressed under abiotic stress. In this regard, it resembles that of the null allele of the bZIP transcription factor FlbB. However a DeltaflbB; DeltakapI double mutant exhibited an additive phenotype with totally impaired conidiation, unresponsive to abiotic stress. In contrast to DeltaflbB, the null kapI mutant is not a fluffy-low-bristle expression mutant. Taken together the findings indicate that KapI is required during asexual development, mediating the nuclear transport of factors acting in a different pathway(s) from those involving the upstream developmental activators.