Pathophysiological condition changes the conformation of a flexible FBG-related protein, switching it from pathogen-recognition to host-interaction

► We examine the mechanism of pH & calcium induced M-ficolin (a FREP):CRP interaction. ► We find the C-terminus of M-ficolin FBG domain to be pH and calcium-sensitive. ► Stronger interaction between M-ficolin FBG:CRP occurs under pH 6.5 and 2 mM Ca2+. ► We observe that CRP binds to the pH and calcium-sensitive C-terminus of FBG. ► Low pH and low calcium induce FBG:CRP interaction via H284.