Thermodynamic and kinetic analysis of the LAO binding protein and its isolated domains reveal non-additivity in stability, folding, and function

Abstract

Substrate-binding proteins (SBP) are used by organisms from the three domains of life for transport and signaling. SBPs are composed of two domains that collectively trap ligands with high affinity and selectivity. To explore the role of the domains and the integrity of the hinge region between them in the function and conformation of SBPs, here we describe the ligand binding, conformational stability, and folding kinetics of the Lysine Arginine Ornithine binding protein (LAO) from Salmonella thiphimurium and constructs corresponding to its two independent domains. LAO is a class II SBP formed by a continuous and a discontinuous domain. Contrary to the expected behavior based on their connectivity, the discontinuous domain shows a stable native-like structure that binds L-arginine with moderate affinity, whereas the continuous domain is barely stable and shows no detectable ligand binding.