The recognition of carbohydrates by proteins underlies key cellular processes such as cell communication, host defence, fertilization, development, parasitic infection and tumour metastasis. Lectins are the carbohydrate-binding proteins of non-immune origin found in all types of living organisms that decipher the glycocodes encoded in the structure of glycans attached to soluble and integral cell-membrane glycoconjugates (Gabius & Gabius, 1997
). Mechanisms for sugar recognition in microorganisms, plants and animals have evolved independently in diverse protein frameworks (Elgavish & Shaanan, 1997
). The largest and best characterized lectin family is that from terrestrial plants and accumulating evidence indicates that the vast majority of these lectins can be classified into four large and three small families of structurally and evolutionary related proteins (Van Damme et al.
; see also the 3D Lectin Database at http://webenligne.cermav.cnrs.fr/lectines/
). The molecular structure and functional features of an increasing number of terrestrial plant lectins have been reported (Loris, 2002
). However, and in marked contrast to higher land plant lectins, marine algal lectins have been isolated and characterized at a much lower pace since the first report of haemagglutinating activity in these organisms appeared more than 35 y ago (Boyd et al.
). Moreover, to date biochemical and structural information on algal lectins is scarce and is available from only a few species, mainly owing to difficulties in their isolation and in obtaining sufficient material for study. Therefore, the functional and phylogenetic classification of these lectins remains obscure.
The available biochemical information indicates the existence of lectins belonging to different protein families in marine algae such as the green algae Enteromorpha prolifera
(Ambrosio et al.
) and Ulva pertusa
(Wang et al.
), the red marine algae Bryothamnion triquetrum
(Calvete et al.
), Hypnea japonica
(Hori et al.
), H. musciformis
and H. cervicornis
(Nagano et al.
) and species of the genera Eucheuma
(Kawakubo et al.
) and Ptilota
(Sampaio et al.
). Moreover, the complete amino-acid sequences of only six algal lectins have been determined (BTL, Calvete et al.
; HJA1 and HJA2, Hori et al.
; UPL1, Wang et al.
; HML and HCA, Nagano et al.
) (Fig. 1).
Figure 1 Amino-acid sequences of lectins belonging to two different protein families isolated from the taxonomically related species of red marine algae of the Hypnea and Bryothamnion genera. (a) HML and HCA, lectins from H. musciformis and H. cervicornis, respectively (more ...)
HML (9357 ± 1 Da), a lectin isolated from the red marine alga H. musciformis
, consists of a mixture of a 90-residue polypeptide containing seven intrachain disulfide bonds and two disulfide-bonded fragments generated by cleavage at the bond Arg50–Glu51 (Nagano et al.
) (Fig. 1). The N-terminal (residues 1–47) and C-terminal (residues 48–90) fragments exhibit a large sequence similarity (Fig. 1) and HML can thus be regarded as a mixture of a single chain with internal homology and a two-chain protein of homologous polypeptides (Nagano et al.
). HML exhibits neither discernible amino-acid sequence similarity with nor the cysteine-spacing pattern found in any other known protein structure, strongly indicating that HML belongs to a novel protein (lectin) family (Nagano et al.
). The high cysteine (disulfide bond) content of HML is an unusual feature for lectin structures. Of the plant lectins, the only other known example of cysteine-rich proteins are the chitin-binding lectins, which are made up of hevein domains (Van Damme et al.
). Here, we report the first crystallization and the preliminary X-ray diffraction analysis from a marine alga lectin.