Fungal Lectins. Molecular structure and function of a member of a novel lectin family
Abstract: Lectins defined as non-enzyme, non-immunoglobulin carbohydrate binding proteins, have been found in a wide variety of viruses, bacteria, fungi, plants and animals. Although lectins have been purified from more than 60 different species of fungi, the structure and biological functions of these proteins are not well known. I examined the molecular structure of a lectin (designated AOL) isolated from the soil-living, nematode-trapping fungus Arthrobotrys oligospora and tried to determine its function(s). Like many other fungal lectins, AOL is a saline-soluble, low-molecular-mass protein consisting of two noncovalently associated, identical subunits (16 150 Da). The primary sequence of AOL displayed a large degree of similarity (identity 46%) to that of another saline-soluble, low-molecular-mass lectin (ABL) isolated from the saprophytic mushroom Agaricus bisporus. The primary sequence showed no significant similarity to any other lectins thus far sequenced from animals, plants or other microorganisms. The binding specificities of AOL and ABL were very similar. Both lectins bound to Galb3GalNAca-Ser/Thr and to certain complex N-linked sugars (if linked to proteins), as well as some sulfated glycoconjugates and to two phospholipids. These data show that AOL and ABL belong to a novel lectin family found in fungi. The fact that AOL and ABL are present in an ascomycete and a basidiomycete respectively (divergence 3.8 x 108 years ago) indicates that these proteins have important functions in fungi. The expression of AOL was metabolically and developmentally regulated. AOL can be very abundant (accounting for up to 20% of the total saline-soluble proteins in the mycelium) once the fungus has entered the stationary growth phase, if it is grown in media with low carbon/nitrogen ratios. During the later stages of the fungal infection (after penetration of the cuticle) of nematodes there was a large accumulation of AOL in the mycelium. Immunoelectron microscopy studies revealed AOL to be very abundant in the cytoplasm and nucleus of the fungal mycelium. The results indicate that AOL is a multifunctional protein involved in the assimilation and storage of nutrients, in non-self recognition (e.g defence) and in the organization of an intracellular matrix.
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