Preparation of soluble and functional recombinant HTLV-1 Tax protein using bacterial chaperones.

HTLV-1 viral protein Tax physically interacts with multiple host proteins to disrupt their normal functions, causing debilitating haematological malignancy and inflammatory disorders. The recombinant Tax protein, like other intrinsically disordered proteins, is insoluble when expressed in Escherichia coli due to the inclusion body formation, hampering structural studies of this important viral protein. We report that the bacterial chaperone Trigger Factor significantly enhances the solubility of HTLV-1 Tax fused to maltose-binding protein (MBP) as a solubility tag. Using a hexahistidine tag in addition to MBP enabled a two-step purification to isolate Tax from the MBP moiety after cleaving the fusion protein with HRV 3C protease. The purified recombinant protein activated NF-κB, indicating that the recombinant protein, solubilised by Trigger Factor, was functional. A molecular thermodynamics simulation, using a 3D model generated by AlphaFold2 as the initial state, suggested two putative domains within the Tax protein. These predicted domains could also be expressed and purified in the presence of exogenous Trigger Factor. By enabling the production of full-length and predicted domains of HTLV-1 Tax in the soluble form, this study takes the first step towards structural studies to understand how HTLV-1 Tax mechanistically interacts with various host factors.