Cloning, purification and comparative structural analysis of two hypothetical proteins from Mycobacterium tuberculosis found in the human granuloma during persistence and highly up-regulated under carbon-starvation conditions

Mycobacterium tuberculosis is a successful pathogen largely due to its ability to persist in humans while evading the host immune system. Rv2557 and Rv2558 are two uncharacterized proteins which have been found to be present in the human granuloma along with other important proteins like isocitrate lyase and nitric oxide reductase which are necessary for long-term persistence. The two proteins are up-regulated in in vitro carbon-starvation conditions designed to mimic the latent stage. Genes corresponding to Rv2557 and Rv2558 are found only in Mycobacterium sp. so far and share high sequence identity of 65.4% at the protein level. In the present study we have cloned and purified the proteins as part of a long-term goal to understand their functional roles and importance to the pathogen. We have probed for their biophysical properties. The proteins are monomeric and do not interact with each other as revealed by size-exclusion chromatography and pull-down assays. Circular dichroism experiments involving temperature and chemical denaturation studies demonstrate that Rv2557 is more structured compared to Rv2558, which is surprising, given the high sequence conservation between them. In fact the free energy change (DeltaG(0)) of Rv2557 during guanidium chloride induced denaturation is higher than Rv2558 indicating higher structural stability. The unfolding studies indicate that overall both proteins unfold in a cooperative two state process but adopt different modes of stabilization. The present work sets the stage for further experiments to probe the functions of the proteins.