Crystal structure of a phospholipase A₂ from Bothrops asper venom: Insights into a new putative "myotoxic cluster"

Snake venoms from the Viperidae and Elapidae families often have several phospholipases A₂ (PLA₂s), 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. PLA₂s from these venoms are generally divided into two classes: (i) catalytic PLA₂s (or Asp49-PLA₂s) and (ii) non-catalytic PLA₂-like toxins (or Lys49-PLA₂s). In many Viperidae venoms, a subset of the basic Asp49-PLA₂s displays some functional and structural characteristics of PLA₂-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-PLA₂ isolated from Bothrops asper venom. The structure presents a dimeric conformation that is compatible with that of previous dimers found for basic myotoxic Asp49-PLA₂s and Lys49-PLA₂s 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-PLA₂s, 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-PLA₂s from Viperidae venoms. Such arrangement may represent a novel structural strategy for the mechanism of muscle damage exerted by the group of basic, Asp49-PLA₂s found in viperid snake venoms.