FC0008
Sterile 5 protein (Ste5)  -  Fusion 3 protein (Fus3)

Biological function
Ste5 is a scaffold protein, which binds to the mitogen-activated protein kinase Fus3 in the mating (pheromone) response MAPK cascade in S. cerevisiae.

Structural evidence
Ste5 interacts with Fus3 in a bipartite manner, and the 8 residue linker is invisible in the complex.

Biochemical evidence
The Kd of the 288-316 fragment is 4 μM, which is comparable to other docking peptide. No binding sites have measurable affinity without a linker. Changing the linker size decreases auto-phosphorylation by Fus3, while binding affinity is not affected.

Structure/Mechanism
Ste5 binding activates Fus3 auto-phosphorylation. Ste5 polypeptide binds to both domains of Fus3, inducing a subtle hinge- bending shift. The shift between the kinase domains may increase the flexibility of the activation loop allowing the Tyr side chain to enter the active site, where it can be autophosphorylated.

Mechanism category
tethering

Posttranslational modification
Ste5 alters the catalytic activity of at least one bound kinase and takes part in a negative regulatory loop that appears to decrease output from the pathway.

Significance
The fuzzy linker size and its transient interactions are distinguished in altering Fus3 flexibility and allosterically enhance its autophosphorylation. Fuzziness in Ste5 contributes to both positive and negative regulation of the pathway.