FC0091
Ribosomal protein S6 kinase alpha-1 (RSK1)  -  S100B

Biological function
RSK1 is a MAPK activated protein kinase, which belongs to the calmodulin-dependent kinase (CaMK) superfamily. Binding to S100B negatively affects phosphorylation of RSK1 by ERK. S100B binds to a C-terminal RSK1 segment that is required not only for ERK2 recruitment but also for the autoinhibition of the RSK1 CaMK-type domain. Thus S100B not only directly interferes with the assembly of the ERK2-RSK1 heterodimeric complex, but also negatively affects the activity of the CaMK-type domain of RSK1.

Domain organization/sequence features
Tandem kinase: N-terminal AGC type kinase domain, C-terminal CaMK type domain.

Structural evidence
Crystal structures of RSK1 constructs with N-terminal truncations at 683, 689, 696 in complex with S100B have been determined. The RSK1 peptides exhibited very different bound conformations, but only very short regions of the 683-735 segment could be localized (697-704, 725-731), the rest of the sequence remained to be disordered in the complex. The polymorphic nature of the bound conformations is also illustrated by the diversity of secondary structures adopted by the linear motif upon interacting with the canonical docking site: coil and helix, respectively. HSQC spectrum of the intervening segment (clamp) remain unchanged upon S100B binding. SAXS data indicate interactions between the N-terminal region of RSK1 and S100B, in addition to a population, where the NTD is extended and is not in contact with the partner.

Biochemical evidence
Affinity of the C terminal region of RSK1 (712-735 AA) for S100B is K_D=4 μM, while the segment including 683-735 AA makes high affinity binding with K_D=40 nM. Mutation of any of the 685-688 residues to Ala decreased binding by one order of magnitude.

Structure/Mechanism
Conformational heterogeneity of RSK1 corresponds to auto-inhibited and released states, depending on the position of the C- terminal αL helix. Kinetic measurements show that the S100B-inhibited state corresponds ~44% of the population.

Mechanism category
conformational selection, tethering

Significance
Fuzziness enables and allosteric inhibition of S100B by shifting the bound conformational ensemble of RSK1 from released to auto-inhibited state.

Medical relevance
In malignant melanomas the first phosphorylation event of the MAPK pathway by ERK2 on RSK1 is inhibited by Ca²⁺ -binding S100 proteins.

Submitted by
Laszlo Nyitray   nyitray@elte.hu