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Cytokinesis, as the final step of cell division, plays an important role in fungal growth and proliferation. In the filamentous fungus Aspergillus nidulans, defective cytokinesis is able to induce abnormal multinuclear or nonnucleated cells and then result in reduced hyphal growth and abolished sporulation. Previous studies have reported that a conserved contractile actin ring (CAR) protein complex and the septation initiation network (SIN) signaling kinase cascade are required for cytokinesis and septation; however, little is known about the role(s) of scaffold proteins involved in these two important cellular processes. In this study, we show that a septum-localized scaffold protein paxillin B (PaxB) is essential for cytokinesis/septation in A. nidulans. The septation defects observed in a paxBdeletion strain resemble those caused by the absence of another identified scaffold protein, α-actinin (AcnA). Deletion of α-actinin (AcnA) leads to undetectable PaxB at the septation site, whereas deletion of paxBdoes not affect the localization of α-actinin at septa. However, deletion of either α-actinin (acnA) or paxBcauses the actin ring to disappear at septation sites during cytokinesis. Notably, overexpression of α-actinin acnApartially rescues the septum defects of the paxBmutant but not vice versa, suggesting AcnA may play a dominant role over that of PaxB for cytokinesis and septation. In addition, PaxB and α-actinin affect the septal dynamic localization of MobA, a conserved component of the SIN pathway, suggesting they may affect the SIN protein complex function at septa. Protein pull-down assays combined with liquid chromatography–mass spectrometry identification indicate that α-actinin AcnA and PaxB likely do not directly interact, but presumably belong to an actin cytoskeleton protein network that is required for the assembly and contraction of the CAR. Taken together, findings in this study provide novel insights into the roles of conserved scaffold proteins during fungal septation in A. nidulans.

ABSTRACTThe importance of polarized growth for fungi has elicited significant effort directed at better understanding underlying mechanisms of polarization, with a focus on yeast systems. At sites of tip growth, multiple protein complexes assemble and coordinate to ensure that incoming building material reaches the appropriate destination sites, and polarized growth is maintained. One of these complexes is the polarisome that consists of Spa2, Bud6, Pea2, and Bni1 in Saccharomyces cerevisiae. Filamentous hyphae differ in their development and life style from yeasts and likely regulate polarized growth in a different way. This is expected to reflect on the composition and presence of protein complexes that assemble at the hyphal tip. In this study we searched for polarisome homologues in the model filamentous fungus Aspergillus nidulansand characterized the S. cerevisiaeSpa2 and Bud6 homologues, SpaA and BudA. Compared to the S. cerevisiaeSpa2, SpaA lacks domain II but has three additional domains that are conserved within filamentous fungi. Gene replacement strains and localization studies show that SpaA functions exclusively at the hyphal tip, while BudA functions at sites of septum formation and possibly at hyphal tips. We show that SpaA is not required for the assembly or maintenance of the Spitzenko¨rper. We propose that the core function of the polarisome in polarized growth is maintained but with different contributions of polarisome components to the process.

Growth in Neurospora proceeds by the highly polarised extension of hyphal tips and the formation of new tips (branches). While much attention has been devoted to tip growth, the mechanisms responsible for the decision to form a branch are unknown. One important issue is the degree to which the decision to branch is made independently by each tip, or is under the control of proximal regions of the colony. We address this question by comparing the lengths of branch intervals growing from one branch point. We find significant correlations between such intervals under nearly all circumstances tested. In contrast, we find no significant correlation between branch intervals adjacent in series. The strength of the correlation depends on the qualitative nature of the branches surveyed. Dichotomous/apical branching shows a high degree of correlation while lateral branching shows a weaker but still statistically significant correlation (this difference cannot be explained either by differences between the growth rates or distributions of lengths of branch intervals). We interpret the observed correlations to mean that the decision to form a branch is not independently controlled by the tip where the branch forms. Instead it is determined, at least in part, by factors at or close to the previous branch.