Cytoplasmic Dynamics of the General Nuclear Import Machinery in Apically Growing Syncytial Cells

Abstract

Karyophenns are transporters involved in the bidirectional, selective and active transport of macromolecules through nuclear pores. Importin-beta 1 is the paradigm of karyopherins and together with its cargo-adapter importin-a mediates the general nuclear import pathway. Here we show the existence of different cellular pools of both imporkin-alpha and -beta 1. homologues, KapA and KapB, n the coenocytic ascomycete Aspergillus rildulans. Fluorescence analysis of hap bid and diploid strains expressing KapB::GFP and/or KapA::mRFP showed patches of both karyopherins concurrently translocating long distances in apically-growing cells. Anterograde and retrograde movements allowed those patches to reach cell tips and distal regions with an average speed in the range of mu m/s. This bidirectional traffic required rnicrotubules as well as kiriesin and dynein motors, since it is blocked by benomyl and also by the inactivation of the dynein/dynactin complex through nudA1 or nudK317 mutations. Deletion of Kinesin-3 motor UncA, required for the transport through detyrosinated microtubules, strongly inhibited KapA and KaplE3 movement along hyphae. Overall, this is the first report describing the bidirectional dynamics of the main nuclear import system in coenocytic fungi. A functional link is proposed between two key cellular machines of the filamentous fungal cell: nuclear transport and the tip-growth apparatus