Ion mobility mass spectrometry coupled with rapid protein threading predictor structure prediction and collision-induced dissociation for probing chemokine conformation and stability.

Unique to ion mobility mass spectrometry (IM-MS) is the ability to provide collision cross section (CCS) data and the capacity to delineate any dissociation and/or unfolding of protein complexes. The strong correlation of the experimentally determined CCS with theory is indicative of the retention of native structure in the gas phase, which in turn, qualifies as a means in evaluating the IM-MS data. The assessment of IM-MS data, however, is currently impeded due to the lack of appropriate structural coordinates to use as input in the in silico calculation of theory. To address this issue, this study involves the use of rapid protein threading predictor (RAPTOR) to generate tertiary structures of closely related monomeric chemokines (MCP-1, MCP-3, MCP-4, and eotaxin) and, subsequently, utilize these models to estimate the theoretical values. Experimental CCS of both the model proteins and chemokines correlate well with theory generated by RAPTOR. All conformations for z = 5+ of chemokines fall within theoretical limits. Of the four chemokines, MCP-4 with z = 6+ appears to adopt an extended conformation, while eotaxin gradually unfolds, and the extended structures of MCP-1 and MCP-3 increase in abundance upon activation. Combining RAPTOR with IM-MS and collision-induced dissociation (CID) enables us to interrogate the conformations of homologous proteins with very similar tertiary structures.