Expression, purification and characterization of the IcmG and IcmK proteins of the type IVB secretion system from Coxiella burnetii.

Coxiella burnetii, the causative agent of Q fever, is an intracellular bacterial pathogen. Studies on Coxiella have shown that a type IVB secretion system (T4BSS) contributes to the establishment of the infection by transferring protein molecules. In this report, we focus on two core proteins of the Coxiella T4BSS, namely the IcmG/DotF protein (CBU_1626) and the IcmK/DotH protein (CBU_1628). Here we present a method for the recombinant expression of IcmG and IcmK in E. coli. IcmG was purified by Strep-Tactin affinity chromatography and size exclusion chromatography, while for the purification of IcmK an additional anion exchange chromatography step was introduced. The yields of the purified IcmG and IcmK proteins were 1.2 mg/L and 3 mg/L, respectively. The purified proteins showed predominant band on SDS-PAGE gel of 37 kDa for the IcmG and 40 kDa for the IcmK. Protein folding is confirmed by circular dichroism spectroscopy. The dynamic light scattering experiment indicated that IcmG and IcmK existed in a homogenous form. Further Blue native PAGE indicates the presences of a monomeric form for the IcmK and IcmG. Our work lays the basis for functional exploration and structural determination of IcmG and IcmK proteins of Coxiella's secretion system.