Routes in Innate Immunity Evolution: Galectins and Rhamnose-binding Lectins in AscidiansMarine Proteins and Peptides

The term ‘lectin’ is commonly used to encompass a wide variety of carbohydrate-binding proteins, widely distributed in viruses, prokaryotes and eucaryotes (Vasta & Ahmed, 2008). The first animal lectins were isolated by Noguchi in early 1900 from Limulus polyphemus and Homarus americanus; many years later, Watkins & Morgan (1952) proposed a sugar-specific binding (l-fucose) property for the eel lectin. Animal lectins are grouped in various molecular families, differing in carbohydrate-recognition domain (CRD) structure and organization (Gabius, 1997; Kilpatrick, 2002; Loris, 2002; Vasta et al., 2004). They are involved in a variety of key biological processes, ranging from development (Kaltner & Stierstorfer, 1998; Kilpatrick, 2002) to immune responses (Arason, 1996; Vasta et al., 1994). Protein–carbohydrate interactions are the basis of a mechanism for signaling functions, cell communication and self–non-self recognitions and are critical in the establishment and maintenance of highly specific mutualistic associations in organism–microbe complexes (Sharon & Lis, 1993). In this respect, mutual benefit (symbiosis or commensalism) depends on the maintenance of a tightly regulated balance, whereas colonization of tissues beneficial to the microbe can lead to the loss of host fitness (pathogenesis), unless host-defense responses are able to eliminate the foreignness (Casadevall & Pirofski, 2000). Microheterogeneity, originating from multiple lectin gene copies, allelic variation or post-translational modifications of the gene products, expands the molecular diversity and recognition capabilities. The molecular repertoire may provide a broad non-self-recognition capacity for an efficient innate immune recognition system based on recognition of carbohydrate moieties. Galectins, rhamnose-binding lectins, C-type lectins, fucolectins, P-type lectins and L-type lectins are some examples of animal lectins (Kilpatrick, 2002; L´opez et al., 2011; Shirai et al., 2009).