Hydrophobins: Multipurpose Protein in fungi

Hydrophobins are the low molecular weight (<20kDa) secreted proteins of filamentous fungi. These are characterized by the presence of eight conserved cysteine residues all along the amino acid chain. Based on hydropathy patterns and solubility characteristics, the hydrophobins are grouped into two classes viz., class I hydrophobin and class II hydrophobin. Hydrophobins are detected in saprophytic molds, edible mushrooms, plant pathogens and mutualistic symbionts (mycorrhizae or lichens). These proteins confers the water repellent properties in conidia, hyphae and multicellular structures of fungus and play a role in cell morphogenesis and pathogenicity. (Jagadeesh et al., 2012).

           The rice blast pathogen Magnaporthe grisea was shown to produce a class II hydrophobin- MHP1. The mhp1 mutant conidia were defective in their cellular organelles and losing viability rapidly. Conidia of such mutant exhibited reduced ability to infect and colonize a susceptible rice cultivar indicating that MHP1 has essential role in surface hydrophobicity and infection related fungal development and is required for pathogenicity of Magnaporthe grisea (Soonok Kim et al., 2005).

          The chestnut blight pathogen Cryphonectria parasitica was shown to produce a class II hydrophobin-cryparin and are found only in the fruiting bodies of the fungus. Cryparin mutant strains were unable to produce fruiting bodies. Eventhough produced, were unable to erupt through the bark of the tree indicating that cryparin plays an essential role in the eruption of fruiting bodies through the bark of its host ( Pam kazimierczak et al., 2005).

          The entomogenous filamentous fungus, Beauveria bassiana expresses two hydrophobin genes, hyd1 and hyd2, involved in cell surface hydrophobicity, adhesion, virulence and to constitute the protective spore coat structure known as the rodlet layer.  The hyd1 mutant strains shown decreased spore hydrophobicity, loss of water mediated dispersal, changes in surface carbohydrate epitopes and b-1,3-glucan distribution, lowered virulence in insect bioassays, but no effect on adhesion. In contrast, the hyd2 mutant strains shown both decreased cell surface hydrophobicity and adhesion, but no effect on virulence indicating their involvement in fungal spore coat rodlet layer assembly which play a role in pathogenesis (Shizhu Zhang et al., 2011).

Cerato-ulmin, a hydrophobin secreted by the causal agents of Dutch elm disease, is a parasitic fitness factor. Dutch elm disease is caused by the aggressive Ophiostoma novo-ulmi and the nonaggressive O. ulmi. Both secrete the protein cerato-ulmin (CU). To determine what role CU plays in the pathology of Dutch elm disease, we constructed a CU overexpression mutant of the nonaggressive O. ulmi H5. Stable integration of a single copy of the cu gene from the aggressive O. novo-ulmi into the genome of the nonaggressive isolate resulted in increased secretion of CU protein (Bradley temple et al.,1997).

          Fungi causes many economically important crop diseases and responsible for significant economic losses. They are endovored with many chemical weapons to cause diseases. Hydrophobins are one such class of chemicals involved in giving cell surface hydrophobicity, appressorium formation, conidiogenesis, virulence and to constitute rodlet layer around hyphae to protect against desiccation. Hence detailed study of hydrophobin structure, expression pattern and their functions will help in developing novel management strategies to manage fungal diseases.

SELECTED REFERENCES:

Pam Kazmierczak, Dae Hyuk Kim and Neal K. Van Alfen., 2005, A hydrophobin of the chestnut blight fungus, Cryphonectria parasitica, is required for stroma pustule eruption. Eukaryot cell, 4(5): 931-936.

Shizhu Zhang, Yu Xian Xia, Byong Kim and Nemat O. Keyhani.,2011, Two hydrophobins are involved in fungal spore coat rodlet layer assembly and each play distinct roles in surface interactions, development and pathogenesis in the entomopathogenic fungus, Beauveria bassiana. Mol. Microbiol., 80(3): 811–826.

Soonok Kim, Pyung Ahn, Hee-Sool Rho and Yong-Hwan Lee., 2005, MHP1, a Magnaporthe grisea hydrophobin gene, is required for fungal development and plant colonization. Mol. Microbiol., 57(5): 1224-1237.