Endocytosis in the shiitake mushroom Lentinula edodes and involvement of GTPase LeRAB7

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

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.

The AlRabring7 E3-Ub-ligase mediates AlRab7 ubiquitination and improves ionic and oxidative stress tolerance in Saccharomyces cerevisiae

The maintenance of ROS homeostasis, membrane biogenesis and recycling of molecules are common stress responses involving specific and complex regulatory network. Ubiquitination is an important and common mechanism which facilitates environmental adaptation in eukaryotes. In the present study we have cloned the AlRabring7, an E3-Ub-ligase, previously identified as AlRab7 interacting partner. The role of AlRabring7 for ubiquitinating AlRab7 and facilitating stress tolerance is analysed. The AlRabring7, with an open-reading frame of 702 bp encodes a protein of 233 amino acids, with RING-HC domain of 40 amino acids. In silico analysis shows that AlRabring7 is a C3HC4-type RING E3 Ub ligase. The protein - protein docking show interaction dynamics between AlRab7-AlRabring7-Ubiquitin proteins. The AlRab7 and AlRabring7 transcript showed up-regulation in response to different salts i.e: NaCl, KCl, CaCl₂, NaCl + KCl, NaCl + CaCl₂, imposing ionic as well as hyperosmotic stress, and also with oxidative stress by H₂O₂ treatment. Interestingly, the AlRabring7 showed early transcript expression with maximum expression in shoots on combinatorial stresses. The AlRab7 showed delayed and maximum expression with NaCl + CaCl₂ stress treatment. The AlRab7 complements yeast ypt7Δ mutants and restored the fragmented vacuole. The in vitro ubiquitination assay revealed that AlRabring7 function as E3 ubiquitin ligase and mediates AlRab7 ubiquitination. Overexpression of AlRab7 and AlRabring7 independently and when co-transformed enhanced the growth of yeast cells during stress conditions. Further, the bimolecular fluorescence complementation assay shows the in planta interaction of the two proteins. Our results suggest that AlRab7 and AlRabring7 confers enhanced stress tolerance in yeast.