Thieving phosphates

ISAS scientists examine an important signalling mechanism in cells

Phosphorylation is one of the most important signalling mechanisms between cells, but it is still largely unknown what happens on the atomic level when a phosphate residue is attached to a protein. A new paper in the Journal PNAS, to which ISAS has contributed, now sheds some light on the question. On the important Raf Kinase Inhibitor Protein (RKIP), the researchers observed that phosphorylation breaks a salt bridge inside the protein, leading to local conformational changes in the molecule and thus modifying the interaction surfaces with which RKIP gets in contact with other proteins.

RKIP is a so-called inhibitor: a protein that inhibits the activity of other proteins. In its unphosphorylated stage, it blocks a kinase called Raf-1, but when a phosphate residue is attached to it, it changes its function and inhibits the G Protein-Coupled Receptor Kinase (GRK) instead. As both kinases play a crucial role in cell signalling, RKIP acts as an important switch – causing correspondingly large damage in cells when its function is impaired. Kristina Lorenz and her team from the Cardiovascular Pharmacology group at ISAS, who were involved in the current PNAS study, have been investigating the role of RKIP in cardiovascular diseases for several years. Other studies suggest that it is involved in Alzheimer’s disease, brain development, diabetes, and cancer as well.

Like all proteins, RKIP is a long chain of amino acids. Due to physical effects, this chain folds in a specific manner to form a three-dimensional structure that gives each protein its distinct function, for instance as support or transport structures, signal transmitter, molecular scissors, or accelerators of chemical reactions. When a protein changes its form, it also changes its function: In the case of RKIP, phosphorylation and the resulting conformational change trigger its switch from one kinase to another. This molecular switch has long been known, but the PNAS study shows for the first time in detail how and where RKIP changes its form. The authors call this local mechanism a “saltbridge theft” because the phosphate residue somewhat snatches the involved salt bridge away from its usual binding partners.

Moreover, the scientists were able to show that this theft mechanism is not restricted to RKIP, but seems to be an evolutionary old, well-conserved mechanism that is applied in many cases of protein phosphorylation. This also partly explains why protein phosphorylation does not always need to occur directly at the interaction surface of a protein, but can also induce conformational changes over large distances in a protein.

The paper titled „A conserved salt bridge competition triggered by phosphorylation regulates the protein interactome” was published online in the Early Edition of PNAS on 5 December 2017: http://www.pnas.org/content/early/2017/12/04/1711543114.abstract