True venom lectin family (C-type lectin)

General Activity
C-type lectins are non-enzymatic proteins that bind carbohydrate in a calcium-dependent manner. They are found extensively throughout plant and animal species, including venom secretions. Lectins bind carbohydrate groups through a combination of hydrogen bonding, van der Waals interactions and hydrophobic interactions. Importantly, this binding is reversible and does not result in modification or hydrolysis of the saccharide group.

In venoms, lectins are mostly secreted by snakes, but some of them are also found in fish. In snake venom, lectins are classified into true C-type lectins (CTLs) that correspond to the above description, and C-type lectin-like proteins (also known as CLPs and currently termed snaclecs ) that lack the sugar-binding loop, and consequently do not bind sugars.

The function of true C-type lectins in venom is not well understood, but they have been described as exhibiting a wide variety of biological effects such as hemagglutination, mitogenic activity, platelet aggregation, edema, elevated vascular permeability, renal effects, hypotension, cytotoxicity toward cell lines and modulation of calcium release from skeletal muscle sarcoplasmic reticulum (see references in Stephen et al., 2011 ).

Sequence characteristics
They possess a carbohydrate recognition domain (CRD) that mostly binds galactose but also mannose (Stephen et al., 2011 ). The recognition domain for these two sugars has been described by Drickamer, 1992 and consists of a key tripeptide motif. The variation of this motif has been shown to have significant impact upon functionality with EPN conferring mannose binding ability, whereas QPD confers galactose binding. New variants of this motif containing EAP, QAP, and LTD have also been recently described, but their carbohydrate specificity has not been studied (Fry et al.,2008 ).

Structure
The vast majority of true snake venom C-lectins characterized to date have been found to exist as homodimeric proteins, with the two subunits linked by a single disulfide bond (Abreu et al., 2006 ).