HDX-ESI-MS Reveals Enhanced Conformational Dynamics of the Amyloidogenic Protein β(2)-Microglobulin upon Release from the MHC-1

The light chain of the major histocompatibility complex class 1 (MHC-1), the protein β(2)-microglobulin (β(2)m), has amyloidogenic properties that arise only upon its dissociation from the MHC-1. Here hydrogen/deuterium exchange electrospray ionization mass spectrometry (HDX-ESI-MS) has been used to compare the solution dynamics of β(2)m in its MHC-1 bound state compared with those of β(2)m as a free monomer. The capability of tandem mass spectrometry to dissociate the MHC-1 into its individual constituents in the gas phase following deuterium incorporation in solution has permitted the direct observation of the exchange properties of MHC-1 bound β(2)m for the first time. The HDX-ESI-MS data show clearly that the H→D exchange of MHC-1 bound β(2)m follows EX2 kinetics and that about 20 protons remain protected from exchange after 17 days. Free from the MHC-1, monomeric β(2)m exhibits significantly different HDX behavior, which encompasses both EX1 and EX2 kinetics. The EX2 kinetics indicate a tenfold increase in the rate of exchange compared with MHC-1 bound β(2)m, with just 10 protons remaining protected from EX2 exchange and therefore exchanging only via the EX1 mechanism. The EX1 kinetics observed for unbound β(2)m are consistent with unfolding of its exchange-protected core with a t(1/2) of 68 min (pH 7, 37° C). Thus, upon dissociation from the stabilizing influence of the MHC-1, free β(2)m becomes highly dynamic and undergoes unfolding transitions that result in an aggregation-competent protein.