Crystal structure of the response regulator spr1814 from Streptococcus pneumoniae reveals unique interdomain contacts among NarL family proteins

• The crystal structure of full-length unphosphorylated spr1814 was determined.
• Spr1814 belongs to the NarL subfamily of signal transduction response regulators.
• The two molecules in the asymmetric unit display different conformations.
• One adopts a typical inactive conformation and the other shows an intermediate state.
• Suggesting conformational changes could occur upon activation.

Spr1814 belongs to the NarL/FixJ subfamily of signal transduction response regulators (RR), and has been predicted to regulate the neighboring ABC transporter, which translocates antibiotic molecules in Streptococcus pneumoniae. Here, we report the crystal structure of full-length unphosphorylated spr1814 at 1.7 Å resolution. The asymmetric unit contains two spr1814 molecules, which display very different conformations. Through comparisons with other RRs structures, we concluded that one molecule adopts a general inactive conformation, whereas the other molecule adopts an intermediate conformation. The superposition of each molecule showed that rotational change of the effector domain occurred in intermediate conformational state, implying that domain rearrangement could occur upon phosphorylation.