Comparative Analyses of the Conformational Dynamics Between the Soluble and Membrane-Bound Cytokine Receptors.

Cytokine receptors receive extracellular cues by binding with cytokines to transduce a signaling cascade leading to gene transcription in cells. Their soluble isoforms, functioning as decoy receptors, contain only the ectodomain. Whether the ectodomains of cytokine receptors at the membrane exhibit different conformational dynamics from their soluble forms is unknown. Using Stimulation-2 (ST2) as an example, we performed microsecond molecular dynamics (MD) simulations to study the conformational dynamics of the soluble and the membrane-bound ST2 (sST2 and ST2). Combined use of accelerated and conventional MD simulations enabled extensive sampling of the conformational space of sST2 for comparison with ST2. Using the interdomain loop conformation as the reaction coordinate, we built a Markov State Model to determine the slowest implied timescale of the conformational transition in sST2 and ST2. We found that the ectodomain of ST2 undergoes slower conformational relaxation but exhibits a faster rate of conformational transition in a more restricted conformational space than sST2. Analyses of the relaxed conformations of ST2 further suggest important contributions of interdomain salt-bridge interactions to the stabilization of different ST2 conformations. Our study elucidates differential conformational properties between sST2 and ST2 that may be exploited for devising strategies to selectively target each isoform.