RKIP structure drives its function: a three-state model for regulation of RKIP

Abstract

Raf kinase inhibitory protein (RKIP) is a highly conserved regulator of many signaling networks whose loss or inactivation can lead to a variety of disease states. The multifaceted roles played by RKIP are enabled by an allosteric structure that is controlled through phosphorylation of RKIP and dynamics in the RKIP pocket loop. Perhaps the most striking feature of RKIP is that it can assume multiple functional states. Specifically, phosphorylation redirects RKIP from a state that binds and inhibits Raf-1 to a state that binds and inhibits GRK2. Recent evidence suggests the presence of a third functional state that facilitates RKIP phosphorylation. Here, we present a three-state model to explain the RKIP functional switch and discuss the role of the pocket loop in regulating RKIP activity.

FIG. 1

Regulation of the RKIP Functional switch. Non-phosphorylated RKIP binds and inhibits Raf, effectively down-regulating ERK and thereby inhibiting metastasis. Phosphorylation of RKIP S153 by PKC switches RKIP function from Raf inhibition to GRK2 inhibition, effectively upregulating PKA, an important kinase involved in heart disease. We propose that PKC interacts with an intermediary state dubbed RKIP^Kin rather than with the RKIP^Raf state directly.

FIG. 2

Important structural features of RKIP. The RKIP binding pocket (circled) was originally implicated in binding PE. S153, the site of PKC phosphorylation, does not interact with the pocket loop directly. Nevertheless, the pocket loop mutant P74L exhibits additional loop dynamics and increased S153 phosphorylation.

FIG. 3

Residues perturbed by P74L. An HSQC overlay for WT (dark) and P74L (light) RKIP illustrates the small yet significant changes in chemical environment for residues in and around the pocket loop. Each peak corresponds to a hydrogen-nitrogen pair with known identity.