Negative Cooperativity in a Polybivalent Complex Allows the Prevalence of a Partial Bound State.

The word "multivalency" carries with it implications of stabilization and increased affinities, however sometimes evolution has opted for multivalent strategies which imperfectly or even negatively impact the affinity of binding interactions. The dynein intermediate chain (IC) binding to TcTex1 and LC8 provides a system for investigating how biological systems may induce such imperfection and utilize it for biological function. LC8 and Tctex1 are homologous proteins, both fold as rotationally symmetric intertwined homodimers which bind to two copies of a client protein, enabling dimerization of the client. LC8 is known to bind over 100 different proteins and has been shown to bind it clients cooperatively, with the second client binding with higher affinity than the first. It is not known whether Tctex1 binding is cooperative, but it does bind to several different partners. Along IC, the LC8 and Tctex1 binding sites are close to one another resulting in additional cooperativity effects. Herein we successfully model the 7-state model of IC/LC8/Tctex1 binding, thereby extracting more information about biologically relevant transitions than have been previously accessed. Our analysis uses Bayesian inference to combine the results from 39 different isotherms to obtain narrow distributions for thermodynamic values. We report the first measurement of cooperativity across subsequent bindings to TcTex1, along with the weakest known cooperativity across LC8 binding. Significantly, we also show anti-cooperativity between LC8 and TcTex1 when binding together. Using the derived free energies, concentrations of the different species illustrates the relatively high concentration of half-bound IC implying a functional role for this state and motivations for biology to employ two weakly dimerizing protein partners rather than one strongly dimerizing partner.