The dynamic behaviour of microtubules and their contributions to hyphal tip growth in Aspergillus nidulans

Abstract

Creating and maintaining cell polarity are complex processes that are not fully understood. Fungal hyphal tip growth is a highly polarized and dynamic process involving both F-actin and microtubules (MTs), but the behaviour and roles of the latter are unclear. To address this issue, MT dynamics and subunit distribution were analysed in a strain of Aspergillus nidulans expressing GFP-a-tubulin. Apical MTs are the most dynamic, the bulk of which move tipwards from multiple subapical spindle pole bodies, the only clear region of microtubule nucleation detected. MTs populate the apex predominantly by elongation at rates about three times faster than tip extension. This polymerization was facilitated by the tipward migration of MT subunits, which generated a tip-high gradient. Subapical regions of apical cells showed variable tubulin subunit distributions, without tipward flow, while subapical cells showed even tubulin subunit distribution and low MT dynamics. Short MTs, of a similar size to those reported in axons, also occasionally slid into the apex. During mitosis in apical cells, MT populations at the tip varied. Cells with less distance between the tip and the first nucleus were more likely to loose normal MT populations and dynamics. Reduced MTs in the tip, during mitosis or after exposure to the MT inhibitor carbendazim (MBC), generally correlated with reduced, but continuing growth and near-normal tip morphology. In contrast, the actin-disrupting agent latrunculin B reduced growth rates much more severely and dramatically distorted tip morphology. These results suggest substantial independence between MTs and hyphal tip growth and a more essential role for F-actin. Among MT-dependent processes possibly contributing to tip growth is the transportation of vesicles. However, preliminary ultrastructural data indicated a lack of direct MT-organelle interactions. It is suggested that the population of dynamic apical MTs enhance migration of the 'cytomatrix', thus ensuring that organelles and proteins maintain proximity to the constantly elongating tip.

In vivo analysis of the functions of γ-tubulin-complex proteins

“…All GCPs localized to septa, but the fluorescence of GCPD-GCPF was barely above background levels. These data are consistent with previous findings that the SPB is the major MTOC throughout the cell cycle (Horio and Oakley, 2005;Sampson and Heath, 2005) and with the finding that microtubules might sometimes grow from septa (Konzack et al, 2005). Konzack and colleagues have suggested that an MTOC might be present at the hyphal apex, on the basis of observations of the movement of a GFP-kinesin-like protein under control of an inducible promoter (Konzack et al, 2005); however, other, more direct, studies (Horio and Oakley, 2005;Sampson and Heath, 2005) have not revealed an apical MTOC and we find no localization of GCPs at the apex.…”

Growth rate of Aspergillus nidulans hyphae is independent of a prominent array of microtubules

“…Filamentous actin arrays are concentrated in areas of active cell extension and cell wall deposition (Heath 1990b(Heath , 1995Heath et al 2000). In Aspergillus nidulans, cytoplasmic actin arrays have documented roles in hyphal extension and in morphogenesis (Harris et al 1994;Torralba et al 1998;Sampson and Heath 2005), septation (Momany and Hamer 1997), and mitochondrial motility (Suelmann and Fischer 2000a). A class I myosin is essential in A. nidulans and is enriched at hyphal tips (McGoldrick et al 1995).…”

The important role of actinin-like protein (AcnA) in cytokinesis and apical dominance of hyphal cells in Aspergillus nidulans

“…They can modulate second messenger signalling pathways, providing intracellular trafficking, and organizing communication networks between inter-and intracellular components Heasman & Ridley, 2008;Sampson & Heath, 2005;Xiang & Plamann, 2003). The actin cytoskeleton is involved in many processes in eukaryotic cells, including interaction with a wide variety of actin-binding proteins such as the actin-capping proteins, the actin filament nucleators, and the actin crosslinking proteins (Santella et al, 2008;Schafer & Schroer, 1999;Schroer et al, 1994;Sjoblom et al, 2008).…”