Lectins

Lectins are carbohydrate-binding proteins which are found in a variety of species ranging from prokaryotes to corals, algae, fungi, plants, invertebrates, and vertebrates. Due to their specific carbohydrate-binding properties, they are highly involved in crucial biological processes such as host-pathogen interaction, cell-cell communication, induction of intracel-lular signaling cascades, and cell targeting. In this regard, lectins are showed to have potential to block the binding between HIV-1 and host cells, preventing the viral infection and dissemination. Evidently, HIV-1 envelope glycoprotein gp120 is extensively glycosylated with numerous N-linked glycosylation sites. Glycans which are seated in these glycosyla-tion sites are rich in mannose and can easily serve as ligands for lectins. The importance of gp120 in viral infection of the target cell makes this protein suitable target for anti-HIV treatment or prophylaxis. In this manner, there are several types of lectins which were isolated from marine sources with potential anti-HIV activity (Sato and Hori, 2009).

Griffithsin is a novel lectin isolated from the red algae Griffithsia sp. with a molecular weight of 12.7 kDa. This 121 amino acid protein is reported to display promising anti-HIV activity (Mori et al., 2005). Griffithsin is completely novel with no cysteine residues and does not have any homology to any of the proteins or translated nucleotide sequences. Studies showed that Griffithsin potently prevented the T-lymphoblastic cells from the cytopathic effects of both laboratory strains and clinical primary isolates of HIV-1. It was also exhibited to be active against both T-cell tropic and macrophage-tropic strains of HIV-1 at concentrations as low as 0.043 pM. More importantly, Griffithsin blocked cell-cell fusion between chronically infected and uninfected cells at subnanomolar concentrations without any cytotoxic effect. In connection with predicted mode of action, this lectin disturbed the binding of CD4 host cell membrane receptor to gp120 in a glycosylation-dependent manner and prevented HIV-1 infection. On the other hand, gp120-Griffithsin bond was inhibited by higher glucose and mannose but not by galactose, xylose, or sialic acid-containing glycoproteins. In a structure-activity relationship perspective, the linker sequences of Griffithsin, Gly-Gly-Ser-Gly-Gly is credited to its unusually potent activity.

In a recent study, a high-mannose-binding lectin (BCA) is isolated from green alga Boodlea coacta with potent antiviral activity against HIV-1 and influenza viruses (Sato et al., 2011). Carbohydrate-binding specificity determination of BCA evidently showed that this lectin has strong specificity for a1-2 linked mannose at nonreducing terminal. The potent anti-HIV-1 activity of BCA was easily predicted from carbohydrate-binding propensity and similarity with formerly reported antiviral lectins. Studies showed that BCA inhibited the HIV-1 infection with EC50 value of 8.2 nM. In addition, surface plasmon resonance analysis reported a high affinity between BCA and the HIV envelope glycoprotein gp120 with an association constant of 3.71 x 108 M~1.

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