Fungi have three tetraspanin families with distinct functions

Background: Tetraspanins are small membrane proteins that belong to a superfamily encompassing 33 members in human and mouse. These proteins act as organizers of membrane-signalling complexes. So far only two tetraspanin families have been identified in fungi. These are PlsI, which is required for pathogenicity of the plant pathogenic ascomycetes, Magnaporthe grisea, Botrytis cinerea and Colletotrichum lindemuthianum, and Tsp2, whose function is unknown. In this report, we describe a third family of tetraspanins (Tsp3) and a new family of tetraspanin-like proteins (TpII) in fungi. We also describe expression of some of these genes in M. grisea and a basidiomycete, Laccaria bicolor, and also their functional analysis in M. grisea. Results: The exhaustive search for tetraspanins in fungal genomes reveals that higher fungi (basidiomycetes and ascomycetes) contain three families of tetraspanins (PlsI, Tsp2 and Tsp3) with different distribution amongst phyla. PlsI is found in ascomycetes and basidiomycetes, whereas Tsp2 is restricted to basidiomycetes and Tsp3 to ascomycetes. A unique copy of each of PLSI and TSP3 was found in ascomycetes in contrast to TSP2, which has several paralogs in the basidiomycetes, Coprinus cinereus and Laccaria bicolor. A tetraspanin-like family (TpII) was also identified in ascomycetes. Transcriptional analyses in various tissues of L. bicolor and M. grisea showed that PLS1 and TSP2 are expressed in all tissues in L. bicolor and that TSP3 and TPL1 are overexpressed in the sexual fruiting bodies (perithecia) and mycelia of M. grisea, suggesting that these genes are not pseudogenes. Phenotypic analysis of gene replacementmutants Delta tsp3 and Delta tpII of M. grisea revealed a reduction of the pathogenicity only on rice, in contrast to Delta plsI mutants, which are completely non-pathogenic on barley and rice. Conclusion: A new tetraspanin family ( Tsp3) and a tetraspanin-like protein family (TpII) have been identified in fungi. Functional analysis by gene replacement showed that these proteins, as well as PlsI, are involved in the infection process of the plant pathogenic fungus M. grisea. The next challenge will be to decipher the role(s) of tetraspanins in a range of symbiotic, saprophytic and human pathogenic fungi.