Crystal structure of a phospholipase A from Bothrops asper venom: Insights into a new putative "myotoxic cluster".

Snake venoms from the Viperidae and Elapidae families often have several phospholipases A (PLAs), which may display different functions despite having a similar structural scaffold. These proteins are considered an important target for the development of drugs against local myotoxic damage because they are not efficiently neutralized by conventional serum therapy. PLAs from these venoms are generally divided into two classes: (i) catalytic PLAs (or Asp49-PLAs) and (ii) non-catalytic PLA-like toxins (or Lys49-PLAs). In many Viperidae venoms, a subset of the basic Asp49-PLAs displays some functional and structural characteristics of PLA-like proteins and group within the same phylogenetic clade, but their myotoxic mechanism is still largely unknown. In the present study, we have crystallized and solved the structure of myotoxin I (MT-I), a basic myotoxic Asp49-PLA isolated from Bothrops asper venom. The structure presents a dimeric conformation that is compatible with that of previous dimers found for basic myotoxic Asp49-PLAs and Lys49-PLAs and has been confirmed by other biophysical and bioinformatics techniques. This arrangement suggests a possible cooperative action between both monomers to exert myotoxicity via two different sites forming a putative membrane-docking site (MDoS) and a putative membrane disruption site (MDiS). This mechanism would resemble that proposed for Lys49-PLAs, but the sites involved appear to be situated in a different region. Thus, as both sites are close to one another, they form a "myotoxic cluster", which is also found in two other basic myotoxic Asp49-PLAs from Viperidae venoms. Such arrangement may represent a novel structural strategy for the mechanism of muscle damage exerted by the group of basic, Asp49-PLAs found in viperid snake venoms.