The tetrameric pheromone module SteC‐MkkB‐MpkB‐SteD regulates asexual sporulation, sclerotia formation and aflatoxin production inAspergillus flavus

Dean Frawley,Berl Oakley,Nancy P Keller,Özgür Bayram,Alastair B Fleming,Claudio Greco,Mohamed M Alhussain

doi:10.1111/cmi.13192

Abstract

For eukaryotes like fungi to regulate biological responses to environmental stimuli, various signalling cascades are utilized, like the highly conserved mitogen‐activated protein kinase (MAPK) pathways. In the model fungus Aspergillus nidulans, a MAPK pathway known as the pheromone module regulates development and the production of secondary metabolites (SMs). This pathway consists five proteins, the three kinases SteC, MkkB and MpkB, the adaptor SteD and the scaffold HamE. In this study, homologs of these five pheromone module proteins have been identified in the plant and human pathogenic fungus Aspergillus flavus. We have shown that a tetrameric complex consisting of the three kinases and the SteD adaptor is assembled in this species. It was observed that this complex assembles in the cytoplasm and that MpkB translocates into the nucleus. Deletion of steC, mkkB, mpkB or steD results in abolishment of both asexual sporulation and sclerotia production. This complex is required for the positive regulation of aflatoxin production and negative regulation of various SMs, including leporin B and cyclopiazonic acid (CPA), likely via MpkB interactions in the nucleus. These data highlight the conservation of the pheromone module in Aspergillus species, signifying the importance of this pathway in regulating fungal development and secondary metabolism.

Highlights

Filamentous fungi, such as the genus Aspergillus, are ubiquitous in the environment and can exhibit both beneficial and detrimental effects with regard to human and plant health
These results indicate that the SteC, MkkB, MpkB and SteD proteins all contribute to the regulation of asexual sporulation in A. flavus, similar to what is observed in A. nidulans
Due to the defects in development observed in the pheromone module mutant strains, we decided to assess whether these proteins contribute to the regulation of secondary metabolism, similar to what is observed in A. nidulans (Bayram et al, 2012; Frawley et al, 2018)

Summary

| INTRODUCTION

Filamentous fungi, such as the genus Aspergillus, are ubiquitous in the environment and can exhibit both beneficial and detrimental effects with regard to human and plant health. A pathway that is homologous to the yeast Fus pheromone module has been characterized in the model ascomycete fungus Aspergillus nidulans (Bayram et al, 2012; Frawley et al, 2018) This pathway consists the MAPKKK SteC, the MAPKK MkkB, the MAPK MpkB, the adaptor protein SteD and the scaffold protein HamE. This study shows that the tetrameric complex in A. flavus is critical for the regulation of asexual sporulation, sexual sclerotia development and aflatoxin production

| RESULTS

| DISCUSSION

Findings

| EXPERIMENTAL PROCEDURES