Ranjeeta Bhari

Mushroom lectins: Current status and future perspectives

Lectins are nonimmune proteins or glycoproteins that bind specifically to cell surface carbohydrates, culminating in cell agglutination. These are known to play key roles in host defense system and also in metastasis. Many new sources have been explored for the occurrence of lectins during the last few years. Numerous novel lectins with unique specificities and exploitable properties have been discovered. Mushrooms have attracted a number of researchers in food and pharmaceuticals. Many species have long been used in traditional Chinese medicines or functional foods in Japan and other Asian countries. A number of bioactive constituents have been isolated from mushrooms including polysaccharides, polysaccharopeptides, polysaccharide–protein complexes, proteases, ribonucleases, ribosome inactivating proteins, antifungal proteins, immunomodulatory proteins, enzymes, lectins, etc. Mushroom lectins are endowed with mitogenic, antiproliferative, antitumor, antiviral, and immunestimulating potential. In this review, an attempt has been made to collate the information on mushroom lectins, their blood group and sugar specificities, with an emphasis on their biomedical potential and future perspectives.

Screening of Aspergillus species for occurrence of lectins and their characterization

Ten species of Aspergillus were screened for occurrence of lectins. Each of the species was investigated for the occurrence of extracellular, surface‐bound and intracellular lectin activities. As many as four species namely, Aspergillus niger, Aspergillus versicolor, Aspergillus rugulosus and Aspergillus nidulans, were found to possess intracellular lectin activities, while none of the species showed extracellular or surface‐bound lectin activities. Each of the lectin was characterized with respect to blood group and carbohydrate specificities. All the lectins were found to agglutinate human erythrocytes, irrespective of their blood group and pig erythrocytes. However, they did not show agglutination with sheep or goat erythrocytes. Of the various carbohydrates tested, all lectins were found to be specific for inulin, mucin, asialofetuin, N‐acetyl galactosamine, melibiose, D‐ribose, L‐fucose, D‐arabinose, D‐sucrose and D‐mannitol. The minimum inhibitory concentration of each of the specific sugars was also determined. The lectins were partially purified using ammonium sulfate precipitation technique. Each of the lectin was found to be precipitated at 40–50% saturation of ammonium sulfate, yielding about 80% of lectin activity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Current trends of lectins from microfungi

Lectins are widespread in nature and have been isolated from plants, animals, microorganisms, and viruses. Although several lectins have been reported from microfungi, many more genera still remain unexplored and their physiological role is also uncertain. Microfungal lectins show wide disparity regarding their specificity to erythrocytes. Only a few lectins display specificity to particular human blood types. In addition, they also show agglutination to various animal erythrocytes. Many lectins from microfungi exhibit stringent specificity to animal glycoproteins, while a few have much more simplified sugar binding properties. The role of few microfungal lectins in host-parasite interactions, as storage proteins, and in growth and morphogenesis has been proposed. The current review focuses on an overview of lectins from microfungi, their specificity towards erythrocytes and carbohydrates, physicochemical characteristics, and their possible role and applications.

Characteristics of yeast lectins and their role in cell–cell interactions

Lectins are ubiquitous proteins with the ability to induce cell agglutination and, mediate cellular and molecular recognition processes in a variety of biological interactions. Fungi display exquisite specificity for target tissues and attach to host glycoconjugates via these sugar-binding proteins. Although only few reports are available on lectin activity of yeasts, these sugar binding proteins have been embraced for their role in cell flocculation, a commercially beneficial property, that simplifies downstream recovery operations in yeast fermentations. The lectins bind to cell wall mannans of the neighboring cells via hydrogen bonds leading to the formation of cell aggregates which get interrupted in the presence of specific sugars. Attachment of pathogenic yeasts to host cell surface is also a consequence of lectin-mediated recognition process. This review provides a brief overview of yeast lectins, with an insight to lectin-mediated cellular recognition phenomenon in yeasts.

Further screening of Aspergillus species for occurrence of lectins and their partial characterization

Fifteen species of Aspergillus were screened for occurrence of lectins. Nine of them (A. sydowii, A. candidus, A. allahabadi, A. terricola, A. ficuum, A. sparsus, A. carneus, A. pulvinus and A. aculeatus) were found to possess lectin activity. None of the species elaborated lectin in culture supernatant. All the lectins agglutinated rat, pig and rabbit erythrocytes. A. sydowii, A. candidus, A. allahabadi, A. terricola, A. ficuum, A. sparsus, A. carneus and A. aculeatus lectins agglutinated all human type erythrocytes equally, while A. pulvinus lectin specifically agglutinated human type A and O erythrocytes. Neuraminidase and protease treatment to erythrocytes substantially augmented lectin titres manyfold. Lectins showed specificity to mucin and asialofetuin and all of them were specific to L‐arabinose except that of A. carneus. Lectins from A. sydowii, A. ficuum, A. sparsus and A. carneus displayed remarkable specificities to D‐xylose. Maximum lectin activity was expressed by 11 day old cultures of A. sydowii (titre 32), A. ficuum (titre 64) and A. sparsus (titre 1024). Lectins from A. aculeatus, A. candidus and A. terricola were expressed by 7–10 days, 6–9 days and 5–11 days old cultures, respectively. A. allahabadi cultures exhibited maximum lectin activity (titre 32) after 8–10 days of cultivation. A. carneus and A. pulvinus expressed optimal titres of 32 and 8, respectively on the 9th day. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimization of culture conditions and characterization of a new lectin from Aspergillus niger

Lectin activity was determined on solidified medium containing agar and in broth cultures of Aspergillus niger. The fungus was found to express 16 times higher activity in broth cultures, when grown in a medium adjusted to pH 5.5 at 30°C under stationary condition. Lectin activity was found to be expressed by 6-day-old mycelial cultures with maximum activity being expressed on 9th day of incubation. The crude lectin (total titer 1280) was found to be precipitated at 50% saturation of ammonium sulphate with 2.4-fold purifi cation and 83% yield in the precipitate. The partially purifi ed lectin was found to agglutinate all human, rat, mice and pig erythrocytes. It was found to have a strong binding affinity to mucin, asialofetuin and inulin.

Lectin activity in mycelial extracts of Fusarium species

Lectins are non-immunogenic carbohydrate-recognizing proteins that bind to glycoproteins, glycolipids, or polysaccharides with high affinity and exhibit remarkable ability to agglutinate erythrocytes and other cells. In the present study, ten Fusarium species previously not explored for lectins were screened for the presence of lectin activity. Mycelial extracts of F. fujikuroi, F. beomiformii, F. begoniae, F. nisikadoi, F. anthophilum, F. incarnatum, and F. tabacinum manifested agglutination of rabbit erythrocytes. Neuraminidase treatment of rabbit erythrocytes increased lectin titers of F. nisikadoi and F. tabacinum extracts, whereas the protease treatment resulted in a significant decline in agglutination by most of the lectins. Results of hapten inhibition studies demonstrated unique carbohydrate specificity of Fusarium lectins toward O-acetyl sialic acids. Activity of the majority of Fusarium lectins exhibited binding affinity to d-ribose, l-fucose, d-glucose, l-arabinose, d-mannitol, d-galactosamine hydrochloride, d-galacturonic acid, N-acetyl-d-galactosamine, N-acetyl-neuraminic acid, 2-deoxy-d-ribose, fetuin, asialofetuin, and bovine submaxillary mucin. Melibiose and N-glycolyl neuraminic acid did not inhibit the activity of any of the Fusarium lectins. Mycelial extracts of F. begoniae, F. nisikadoi, F. anthophilum, and F. incarnatum interacted with most of the carbohydrates tested. F. fujikuroi and F. anthophilum extracts displayed strong interaction with starch. The expression of lectin activity as a function of culture age was investigated. Most species displayed lectin activity on the 7th day of cultivation, and it varied with progressing of culture age.