Caldalkalibacillus thermarum PhoH2: Solving the solubility puzzle

Abstract

Previous attempts to determine the structure of the protein PhoH2 from Mycobacterium tuberculosis, Mycobacterium smegmatis and Thermobispora bispora have been unsuccessful producing diffraction data to low resolution. The thermophilic organism Caldalkibacillus thermarum was thought to be an advantageous option from which to clone, express and purify PhoH2 from for the purpose of structure determination. PhoH2 consists of two domains, an N terminal PIN domain, known for its toxic properties as part of many toxin-antitoxin systems present in M. tuberculosis, and a C-terminal PhoH domain, an RNA helicase suspected to be involved in phosphate starvation responses. The cloning and expression of three variations of recombinant C. thermarum PhoH2 was successful in Escherichia coli. However the purification of PhoH2 continues to yield insoluble protein, despite the range of purification buffer conditions screened, thus preventing downstream biochemical characterisation and structural investigations. Multiple options exist to overcome this insolubility problem; including alternate plasmids for protein expression and purification that alter the tag location and type such as C-terminal His-tags and alternative fusion tags.