In this study, we have analyzed the dynamic conformations of wild-type NPM1 and M7-NPM using deuterium exchange mass spectrometry, in order to better understand the structural basis for altered oligomer formation. DXMS has been used to study the conformational changes of proteins under various conditions and in combination with a multitude of binding partners and cofactors. This technique measures the exchange of back-bone amide protons for solvent deuterons, and with the aid of protease digestion, maps the accessibility of various regions with peptide-level resolution; in turn, the presence of exchange captured under various conditions often indicates very specific local and global structures. Furthermore, the morphology of the deuterated peptide Salubrinal spectra is determined by both local interactions which affect the accessibility of amide protons, and the global and regional structures which determine the kinetics of catalytic deuteration by hydroxyl ions. The relative kinetics of these multiple processes produce distinct DXMS data. For example, ����unfolding���� proteins by increasing temperature or using higher concentrations of denaturant generate bimodal mass spectra that are classically termed EX1; these data reflect local ����refolding���� rates which are much smaller than rates of catalytic deuteration. Here, we describe the unexpected discovery of EX1 kinetics at a key monomer interface that includes the b-hairpin loop, indicating significant local structural flexibility in wild-type NPM1 under non-denaturing conditions. This interface was an area of important differences between wild-type NPM1 and M7NPM structure, and furthermore, targeted disruption of part of this interface, at the b-hairpin ����latch����, prevented formation of stabilized oligomers. Finally, mutations that affect nucleophosmin oligomer formation also changed recognition and cleavage by SKI II granzyme B. Thus, we present evidence of dynamic structural shifts in NPM1 that significantly impact protein-protein interactions and may represent a target for altering NPM1 function.