Small Angle X‐ray Scattering Study of a Histidine Kinase Embedded in Styrene‐Maleic Acid Copolymer Lipid Particles

Abstract

Histidine kinases constitute an abundant class of membrane receptors present in all domains of life. Many of them are essential for cell growth, survival, or pathogenicity. However, the mechanisms of transmembrane signaling by such sensors are not fully understood due to the scarcity of structural information on full-length proteins. We used styrene–maleic acid (SMA) copolymers to solubilize full-length nitrate/nitrite sensor histidine kinase NarQ from Escherichia coli. The resulting SMA lipid particles (SMALPs) provided a native membrane-like environment for NarQ and remained stable for an extended period of time. We characterized NarQ-containing SMALPs using size-exclusion chromatography and small angle X-ray scattering. Overall, the particles were heterogeneous in size, with the scattering from the smallest ones matching the theoretical scattering from a single NarQ dimer embedded in a SMALP comprising ~150 SMA molecules and ~350 lipids. These results show that SMA can be used to extract full-length histidine kinases from the membrane with efficiency comparable to, and exceeding that of the commonly used detergent dodecyl maltoside (DDM), and the resulting samples are suitable for structural studies.