The anthrax protective antigen is an 83 kDa protein secreted by Bacillus anthracis, and is required for entry of the edema factor and lethal factor, that leads to Anthrax. PA bound to Capillary Morphogenesis Receptor (CMG2) in the host initiates the pathogenesis. Domain 4 has been shown to be important not just in binding to the host cellular receptor, but also in providing protective immunity against anthrax intoxication. The presence of many antigenic epitopes make domain 4 a better candidate vaccine but its usage in vaccine formulations may be compromised due to the thermal stability and its inability to exist in its native state in physiological conditions. An increase in stability of domain 4 may be significant in terms of the development of a better anthrax vaccine. Herein, we use biophysical methods to investigate the stability of isolated Domain IV when bound to CMG2. The implications of our data with respect to anthrax pathogenesis will be discussed.