The long unstructured region of Bcl-xl modulates its structural dynamics.

Bcl-xl protein has a long unstructured loop attached to its structured region which joins two helices. The necessity to have this unstructured segment in Bcl-xl is not yet well understood. To what extent the unstructured segment can influence the dynamics of the structured region of protein, with potential to influence the function, has been investigated in this work. Molecular dynamics simulation and principal component analysis show how the loop affects the internal motions of the protein, particularly its ligand binding pocket. Generally an unstructured region in the structure would promote flexibility resulting entropic stability but in contrary, here it narrows down the conformational space of the structured region of protein that could be hypothesized to impact the functional precision. Effects of the loop propagate to the binding pocket through structural rearrangements of polar side chains. The immediate suspicion of possible impact of phosphorylation to modulate the function of the protein is proven to be a fact, as the phosphorylated S49 and S62 located on the large unstructured region are seen to perturb the electrostatic network of the structure; an observation that validates and clarifies the role of loop as a modulator through biophysical and biochemical mechanisms. Proteins 2017; 85:1567-1579.