Early endocytosis as a key to understanding mechanisms of plasma membrane tension regulation in filamentous fungi

Abstract

Background Information. Two main systems regulate plasma membrane tension (PMT) and provide a close connection between the protoplast and the cell wall in fungi: turgor pressure and the actin cytoskeleton. These systems work together with the plasma membrane focal adhesion to the cell wall and their contribution to fungal cell organization and physiology has been partially studied. However, it remains controversial in model filamentous ascomycetes and oomycetes and even less investigated in filamentous basidiomycetes. Early endocytosis can be used to research the mechanisms regulating PMT since the dynamics of early endocytosis is largely dependent on this tension. Results. This study examined the effects of actin polymerization inhibitors and hyperosmotic shock on early endocytosis and cell morphology in two filamentous basidiomycetes. The main obtained results are: (i) the depolymerisation of F-actin leads to the fast formation of endocytic pits while inhibiting of their scission from the plasma membrane and (ii) the moderate hyperosmotic shock does not affect the dynamics of early endocytosis. These and our other results have allowed suggesting a curtain model for the regulation of PMT in basidiomycetes. Conclusions and significance. According to the proposed curtain model, the PMT in many non-apical cells of hyphae is more often regulated not by turgor pressure but by a system of actin driver cables that are associated with the proteins of the focal adhesion sites. The change in PMT occurs similar to the movement of a curtain along the curtain rod using the curtain drivers. This model addresses the fundamental properties of the fungal structure and physiology. It requires confirmation including the currently technically unavailable high-quality labelling of the actin cytoskeleton of the basidiomycetes.

Lomasomes and other fungal plasma membrane macroinvaginations have a tubular and lamellar genesis

“…These steps correspond to: the stage of formation of endocytic patches (primary vesicles covered in a rigid actin scaffold) and early endosomes; the stage of labeling late endosomes/early vacuoles; and the stage of labeling tonoplasts of vacuoles-lysosomes. Similar results were obtained in our previous studies for Rhizoctonia solani and some species of wood-decaying basidiomycetes (Kamzolkina et al, 2017; Mazheika et al, 2020a, b). We have proposed two important things related to fungal endocytosis and fungal cell organization.…”

“…Our previous publication showed that hyphae of Rhizoctonia solani under hypertonic conditions could shrink in cross section with a loss of up to 15% in hyphae diameter (Mazheika et al, 2020a). The cell wall of fungi is elastic, the protoplast is attached to it from the inside through multiple sites of focal adhesion, therefore, when cells lose water, hyphae, like yeast cells, shrink entirely.…”

“…Tubular invaginations (up to five µm long) at the fluorescent-microscopic level are described for filamentous basidiomycetes in our previous works (Mazheika et al, 2020a;Mazheika, Kamzolkina, 2021). At the electron-microscopic level, invaginating tubes are often presented as curved towards their base and are described not only in filamentous basidiomycetes, but also in yeasts (Smaczynska-de Rooij et al, 2011;Palmer et al, 2015;Schuster, Steinberg, 2020).…”

“…An important result should be added here: mounting the slides with a hyperosmotic medium enhances the formation of all types of macroinvaginations from glomeruli to thick tubes, but partial destruction of F-actin by LatA increases the number of only glomeruli/pendants and small vesicles, without affecting the longitudinal thin tubes, large vesicles and thick tubes. All this can be explained as follows: in freshly prepared microscopic preparations under the conditions of this study, the control over the tension of the PM is performed mainly by the curtain actin subsystem (for the curtain model see Introduction and Mazheika et al, 2020a). The fungus, using the system of actin driver cables, creates a certain tension of the PM, at which only tubular invaginations of small diameter are formed (most likely with the participation of BAR proteins).…”

“…We have proposed two important things related to fungal endocytosis and fungal cell organization. First, based on a number of features of early endocytosis, including the independence of the formation of primary PM invaginations from the actin cytoskeleton, the curtain model of the regulation of PM tension in fungi has been proposed (Mazheika et al, 2020a). Second, two types of fungal endocytosis have been identified (Mazheika et al, 2020b).…”