Diversity of function and higher-order structure within HWE sensor histidine kinases

Abstract

Integral to the protein structure/function paradigm, oligomeric state is typically conserved 31 along with function across evolution. However, notable exceptions such as the hemoglobins 32 show how evolution can alter oligomerization to enable new regulatory mechanisms. Here we 33 examine this linkage in histidine kinases (HKs), a large class of widely distributed prokaryotic 34 environmental sensors. While the majority of HKs are transmembrane homodimers, 35 members of the HWE/HisKA2 family can deviate from this architecture as exemplified by our 36 finding of a monomeric soluble HWE/HisKA2 HK (EL346, a photosensing Light-Oxygen-Voltage 37 (LOV)-HK). To further explore the diversity of oligomerization states and regulation within this 38 family, we biophysically and biochemically characterized multiple EL346 homologs and found a 39 range of HK oligomeric states and functions. Three LOV-HK homologs are primarily dimeric with 40 differing structural and functional responses to light, while two Per-ARNT-Sim (PAS)-HKs 41 interconvert between differentially active monomers and dimers, suggesting dimerization might 42 control enzymatic activity for these proteins. Finally, we examined putative interfaces in a 43 dimeric LOV-HK, finding that multiple regions contribute to dimerization. Our findings suggest 44 the potential for novel regulatory modes and oligomeric states beyond those traditionally defined 45 for this important family of environmental sensors.