Seeking the Roles for Fungal Small-Secreted Proteins in Affecting Saprophytic Lifestyles

Abstract

Small secreted proteins (SSPs) comprise 40-60% of the total fungal secretome and are present in fungi of all phylogenetic groups, representing the entire spectrum of lifestyles. They are characteristically shorter than 300 amino acids in length and have a signal peptide. The majority of SSPs are coded by orphan genes, which lack known domains or similarities to known protein sequences. Effectors are a group of SSPs that have been investigated extensively in fungi that interact with living hosts, either pathogens or mutualistic systems. They are involved in suppressing the host defense response and altering its physiology. Here, we aim to delineate some of the potential roles of SSPs in saprotrophic fungi, that have been bioinformatically predicted as effectors, and termed in this mini-review as "effector-like" proteins. The effector-like Ssp1 from the white-rot fungus Pleurotus ostreatus is presented as a case study, and its potential role in regulating the ligninolytic system, secondary metabolism, development, and fruiting body initiation are discussed. We propose that deciphering the nature of effectorlike SSPs will contribute to our understanding of development and communication in saprophytic fungi, as well as help, to elucidate the origin, regulation, and mechanisms of fungal-host, fungal-fungal, and fungal-bacterial interactions.

At least three families of hyphosphere small secreted cysteine‐rich proteins can optimize surface properties to a moderately hydrophilic state suitable for fungal attachment

“…Recent studies have shown that besides enzymes and small molecules, the secretomes of filamentous and dimorphic fungi can frequently contain a diversity of small secreted cysteine‐rich proteins (SSCPs) unique for fungi (Kubicek et al ., 2011 , 2019 ; Wang et al ., 2019 ; Feldman et al ., 2020 ; Gao et al ., 2020 ; Huber et al ., 2020 ). This diverse group comprises a few distinct protein families, such as hydrophobins (HFBs) and cerato‐platanins (CPs), although most SSCPs remain unclassified.…”

“…Individual fungal genomes can contain from a few dozens of such proteins in yeasts (Wang et al ., 2019 ; Sun et al ., 2020 ) up to several hundred in mushrooms (Zajc et al ., 2013 ; Krizsán et al ., 2019 ). The SSCPs are characterized by their small size (<300 amino acids, a.a.) (Pellegrin et al ., 2015 ; Kubicek et al ., 2019 ), the presence of a relatively short signal peptide (~20 a.a.) (Chen et al ., 2013 ; Kim et al ., 2016 ; Feldman et al ., 2020 ) and cysteine enrichment. The latter feature is crucial for their properties and structure but is frequently ambiguous.…”

“…Probably the most interesting feature of SSCPs is their abundant secretion in liquid medium (up to 20%–30% of the total secretome) (Pellegrin et al ., 2015 ; Feldman et al ., 2020 ; Gao et al ., 2020 ) and the unusually high polymorphism of their primary structures (Gaderer et al ., 2014 ; Krizsán et al ., 2019 ). Some SSCPs have a long evolutionary history of gene duplications (GDs) and lateral gene transfers (LGTs), making them frequent members of the orphomes (the genome fraction comprising orthologue‐less genes) of individual fungal taxa (Przylucka et al ., 2017a ; Feldman et al ., 2019 ; Gao et al ., 2020 ).…”

CgNPG1 as a Novel Pathogenic Gene of Colletotrichum gloeosporioides From Hevea brasiliensis in Mycelial Growth, Conidiation, and the Invasive Structures Development

“…CgNPG1 contained 144 amino acids, of which 10 cysteines accounted for 6.9% and did not have any transmembrane domain except a signal peptide at the N-terminal of 1 to 16 amino acids ( Supplementary Figure 1 ). Usually, the SSPs classified as effectors had the following characteristics: the sequence length of ≤300 amino acids and ≥4 cysteines, and function as key infection factors in suppressing host defense responses and modulating its physiology ( Feldman et al, 2020 ). In our study, CgNPG1 matched the above sequence characteristics and effects in mycelial growth, conidiation, invasive structure development, and pathogenesis ( Figures 3 – 7 ).…”

Cytotoxic Potential from the Extracellular Filtrate from Aspergillus niveus Cultured in YPD Medium against Human Tumor Cells

“…The diversity of the proteins found in the fungal secretome depends on the culture conditions used during the fermentation. In general, 40-60% of this secretome comprises small secreted proteins with less than 300 amino acids [32]. The 12% SDS-PAGE profile obtained for the secreted proteins by the fungus A. niveus cultured for 120 h in different media, revealed the presence of small proteins and differences at the level of gene expression (Fig.…”