An endoplasmic reticulum domain is associated with the polarized growing cells of Podospora anserina hyphae

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Highlights

  • An ER domain is associated with the polarized growing apical cells of hyphae.

  • The apical ER domain consists of dynamic and pleomorphic ER sub-compartments.

  • The apical ER sub-compartments undergo fusion and division events.

  • The apical ER sub-compartments display acropetal and basipetal displacements.

Abstract

The endoplasmic reticulum (ER) is composed of distinct structural domains that perform diverse essential functions, including the synthesis of membrane lipids and proteins of the cell endomembrane system. The polarized growth of fungal hyphal cells depends on a polarized secretory system, which delivers vesicles to the hyphal apex for localized cell expansion, and that involves a polarized distribution of the secretory compartments, including the ER. Here we show that, additionally, the ER of the ascomycete Podospora anserina possesses a peripheral ER domain consisting of highly dynamic pleomorphic ER sub-compartments, which are specifically associated with the polarized growing apical hyphal cells.

Introduction

The endoplasmic reticulum (ER) is composed of different structural domains that perform diverse essential functions, including the synthesis, processing and transport of membrane lipids and proteins of the cell endomembrane system (Schwarz and Blower, 2016, Westrate et al., 2015). In addition, the ER participates in organelle biogenesis (Joshi et al., 2017) and interacts with most other organelles (Wu et al., 2018), which contributes to the regulation of intracellular dynamics. The ER is composed of two major domains, the nuclear envelope and the peripheral ER. The peripheral ER consists of a reticular network of tubules and cisternal sheets that extends throughout the cell and that comprises diverse structural subdomains, whose arrangement is adapted to their specialized functions (Westrate et al., 2015).

The polarized growth mode of fungal hyphae is determined by a secretory system, which drives vesicles to the hyphal tip to deliver membrane lipids and proteins for localized cell surface extension. This process relies on vesicles that transport enzymes involved in cell wall assembly, and is coupled to enzyme endocytic recycling (Hernandez-Gonzalez et al., 2018, Riquelme, 2013, Riquelme et al., 2018, Shaw et al., 2011). Polarized vesicle traffic orchestration involves the Spitzenkörper, a complex structure located in the apex of growing hyphae that is proposed to act as a vesicle supply center that concentrates vesicles for their subsequent delivery to the foremost apical membrane (Riquelme et al., 2018, Riquelme and Sanchez-Leon, 2014). Additionally, the secretory compartments –namely the Golgi cisternae (Pantazopoulou and Penalva, 2009) and the ER (Kimura et al., 2010, Markina-Inarrairaegui et al., 2013, Maruyama et al., 2006, Wedlich-Söldner et al., 2002)– display a polarized distribution along hyphae. Here we show that, in addition, a dynamic peripheral ER domain is specifically associated with the growing apical hyphal cells of the ascomycete Podospora anserina.

Section snippets

Results and discussion

We analyzed the P. anserina ER by studying the localization of an ectopically-expressed ER-targeted GFP (ER-GFP) containing the ER targeting and retention signals of the putative P. anserina ER chaperone BiP/Kar2, a protein that localizes to the ER in filamentous ascomycetes (Maruyama et al., 2006). Consistent with ER localization, ER-GFP localized to a large network of interconnected strands extending throughout hyphae, similar to the ER arrangement of other filamentous fungi (Kimura et al.,

Methods

P. anserina methods, sequences. P. anserina methods and media composition can be consulted at http://podospora.i2bc.paris-saclay.fr. BiP (Pa_4_6420) gene sequence was obtained from the P. anserina genome sequence (Espagne et al., 2008) and is available under GenBank accession number CDP28607.1. BiP signal peptide sequence was predicted on SignalP 4.1 (Nielsen, 2017).

Construction and analysis of ER-targeted GFP. Fusion PCR was used to generate a construct coding for GFP flanked by the ER

Acknowledgements

We are grateful to Fernando Suaste Olmos for technical assistance throughout this research. We thank Sara Schroder for her careful review of the manuscript, and the IFC Imaging Facility for assistance on microscopy. This work was supported by CONACYT, Mexico [CONACYT (FONCICYT)-DFG 277869], and by PAPIIT-DGAPA, UNAM, Mexico [IA203317, IV200519]. AJLF was supported by a scholarship from CONACYT.

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Present address: School of Biomedical Sciences, University of Queensland, Brisbane, QLD 4072, Australia.

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