Nondenaturing Solubilization of Inclusion Bodies from Lactic Acid Bacteria

Since inclusion bodies (IBs) contain an important amount of properly folded and active proteins, their solubilization using nondenaturing conditions to obtain aggregation-prone proteins has gained interest. Through these conditions, the refolding step is no longer required, which avoids the usual protein yield loss after this process. Here, we reveal a simple methodology to obtain pure and active difficult-to-produce proteins using two LPS-free expression systems: Lactococcus lactis and Lactobacillus plantarum. This protocol has proven to be successful to obtain proteins which are labile and prone-to-attach (difficult to be purified from other cytoplasmic proteins) and prone-to-aggregate (difficult to be obtained in their soluble form).

Liquid Native MALDI Mass Spectrometry for the Detection of Protein-Protein Complexes

Native mass spectrometry (MS) encompasses methods to keep noncovalent interactions of biomolecular complexes intact in the gas phase throughout the instrument and to measure the mass-to-charge ratios of supramolecular complexes directly in the mass spectrometer. Electrospray ionization (ESI) in nondenaturing conditions is now an established method to characterize noncovalent systems. Matrix-assisted laser desorption/ionization (MALDI), on the other hand, consumes low quantities of samples and largely tolerates contaminants, making it a priori attractive for native MS. However, so-called native MALDI approaches have so far been based on solid deposits, where the rapid transition of the sample through a solid state can engender the loss of native conformations. Here we present a new method for native MS based on liquid deposits and MALDI ionization, unambiguously detecting intact noncovalent protein complexes by direct desorption from a liquid spot for the first time. To control for aggregation, we worked with HUαβ, a heterodimer that does not spontaneously rearrange into homodimers in solution. Screening through numerous matrix solutions to observe first the monomeric protein, then the dimer complex, we settled on a nondenaturing binary matrix solution composed of acidic and basic organic matrices in glycerol, which is stable in vacuo. The role of temporal and spatial laser irradiation patterns was found to be critical. Both a protein-protein and a protein-ligand complex could be observed free of aggregation. To minimize gas-phase dissociation, source parameters were optimized to achieve a conservation of complexes above 50% for both systems. Graphical Abstract ᅟ.