Light regulates the degradation of the regulatory protein VE-1 in the fungus Neurospora crassa

Abstract

Background Fungi use light as an environmental signal to regulate developmental transitions that are key aspects of their biological cycles and that are also relevant for their dispersal and infectivity as plant or animal pathogens. In addition, light regulates the accumulation of photoprotective pigments, like carotenoids, and other secondary metabolites. Most fungal light responses occur after changes in gene transcription and we describe here a novel effect of light in the regulation of degradation of VE-1, a key component of the velvet complex, in the model fungus Neurospora crassa. The velvet complex is a fungal-specific protein complex that coordinates fungal development, secondary metabolism, and light regulation by interacting with other regulators and photoreceptors and modifying gene expression. Results We have characterized the role of VE-1 during conidiation in N. crassa. In vegetative mycelia, VE-1 is localized in the cytoplasm and nuclei and is required for light-dependent transcription but does not interact with the photoreceptor and transcription factor WC-1. VE-1 is more stable in light than in darkness during asexual development (conidiation). We have shown that this light effect requires the blue-light photoreceptor WC-1. We have characterized the role of the proteasome, the COP9 signalosome (CSN), and the adaptor component of cullin-RING ubiquitin ligases, FWD-1, in the degradation of VE-1. Conclusions We propose that this new effect of light allows the fungal cell to adapt quickly to changes in light exposure by promoting the accumulation of VE-1 for the regulation of genes that participate in the biosynthesis of photoprotective pigments.

Transcriptional Regulation by the Velvet Protein VE-1 during Asexual Development in the Fungus Neurospora crassa

“…2E ), but the accumulation of VE-1 in nuclear fractions was mostly observed after the induction of conidiation in the dark ( Fig. 2D ), as we previously reported in light-exposed cultures ( 29 ). Our observations on the abundances and the nuclear localization of VE-1 and VE-2 suggest that VE-1 is the limiting component of the velvet complex and that the regulation of its abundance and subcellular localization plays a key role in the activity of the velvet complex during asexual development.…”

“…Initial studies have shown that the absence of VE-1 led to a light-dependent shortening of aerial tissue, a mixture of aerial hyphae and developing macroconidia, that required the activity of the photoreceptor WC-1 ( 29 ). This observation, together with the proposed role of the velvet complex in the regulation of asexual development in N. crassa ( 28 ), prompted us to further characterize the roles of VE-1 and VE-2 in the growth of aerial tissue, the amounts of conidia in single and double deletion mutants, and their possible regulation by light.…”

“…The role of the velvet complex in asexual development suggested that it could be present in the nuclei of aerial tissue to regulate transcription. We have previously reported the presence of the velvet complex in the nuclei and cytoplasm of vegetative hyphae ( 28 ) and the accumulation of VE-1 in the nuclei and cytoplasm of aerial tissue in a light-regulated manner ( 29 ). To characterize the accumulation of mRNAs and proteins during conidiation, we collected samples from vegetative mycelia cultured in liquid medium (submerged vegetative mycelia) prior to the induction of conidiation.…”

“…The alterations in aerial hyphal growth and conidiation observed in the Δ ve-1 mutant, together with our previous proposal that VE-1 is required for full light-dependent transcription along with the WCC ( 29 ), prompted us to characterize the role of VE-1 in transcription using RNA-seq experiments. To characterize the transcriptional response to light in vegetative mycelia, cultures of the wild-type and Δ ve-1 strains were grown in liquid medium for 47.5 h in the dark, followed by 30 min of growth in the light or being kept in the dark as a control, prior to mRNA purification from vegetative mycelia (light regulation experiments).…”

“…VE-1 has been observed in the cytoplasm and nuclei of vegetative hyphae, and the nuclear accumulation of VE-1 increases during conidiation. In addition, VE-1 is degraded by the proteasome during conidiation, but the degradation of VE-1 is regulated by light, presumably to allow a quick transcriptional response to changes in light intensity during conidiation ( 29 ). It is not known how VE-1 and the other components of the velvet complex interact and regulate transcription during conidiation.…”