Regulated unfolding of proteins in signaling

Abstract

The transduction of biological signals often involves structural rearrangements of proteins in response to input signals, which leads to functional outputs. This review discusses the role of regulated partial and complete protein unfolding as a mechanism of controlling protein function and the prevalence of this regulatory mechanism in signal transduction pathways. The principles of regulated unfolding, the stimuli that trigger unfolding, and the coupling of unfolding with other well characterized regulatory mechanism are discussed.

Figure 1

p27 as a signaling conduit. Tyrosine phosphorylation-dependent partial unfolding of p27 triggers signal propagation through the length of the protein and regulates its degradation. Step 1 involves phosphorylation of Y88 of p27 that is bound to Cdk/cyclin complexes [Cdk2 (K2)/cyclin A (A) here] by non-receptor tyrosine kinases such as BCR-ABL, Src, Lyn, and Jak2, which ejects Y88 from the ATP binding pocket of Cdk2 and restores partial kinase activity. Following Step 1, Step 2 involves phosphorylation of T187 within the flexible C-terminal domain of p27 by partially active Cdk2 through a pseudo uni-molecular mechanism (indicated by gray arrow). Phosphorylation of T187 creates a phosphodegron signal for ubiquitination of Lysine residues within the p27 C-terminus by the E3 ligase, SCF^Skp2, during Step 3. Finally, during Step 4, ubiquitinated p27 is selectively degraded by the 26S proteasome, leading to the release of fully active Cdk2/cyclin A, which drives progression into S phase of the cell division cycle.

Figure 2

Examples of various regulated unfolding mechanisms involved in signaling. (A) Signal integration in the regulatory mechanism of WASP. In the autoinhibited form, the GBD domain (blue and yellow boxes) is bound to the C-terminal VCA domain (red box), inhibiting the binding of VCA to the Arp2/3 complex (orange box). Binding of Cdc42 GTPase (coral object) to the GBD domain of WASP, which requires partial unfolding and remodeling of this binding site, releases VCA and activates WASP. Phosphorylation of Y291 further stabilizes the active form of WASP. WASP integrates disparate signals (Cdc42 binding and phosphorylation by Lyn) to enable Arp2/3 binding and promote actin polymerization. (B) Metamorphic proteins require partial or global unfolding to interconvert between different tertiary and quaternary structures. (C) Formation of colon mucus through regulated unfolding of MUC2. (D) Functional cycle of the Hsp33 oxidative stress-response chaperone. The sensing domain (green) unfolds in response to oxidative stress, leading to exposure of the substrate binding domain (red) that binds and holds partially unfolded substrates. Restoration of reducing conditions causes the structure of the chaperone to revert to the folded, inactive form, releasing substrates to undergo folding under non-stress conditions.

Figure 3

Several different triggering stimuli mediate protein unfolding and regulate function.