Chemical biology strategies for posttranslational control of protein function

Abstract

A common strategy to understand a biological system is to selectively perturb it and observe its response. Although technologies now exist to manipulate cellular systems at the genetic and transcript level, the direct manipulation of functions at the protein level can offer significant advantages in precision, speed, and reversibility. Combining the specificity of genetic manipulation and the spatiotemporal resolution of light- and small molecule-based approaches now allows exquisite control over biological systems to subtly perturb a system of interest in vitro and in vivo. Conditional perturbation mechanisms may be broadly characterized by change in intracellular localization, intramolecular activation, or degradation of a protein-of-interest. Here we review recent advances in technologies for conditional regulation of protein function and suggest further areas of potential development.

Fig 1

Comparison of naturally occuring and synthetic engineered control processes. a) The central dogma of molecular biology governs the transcription of DNA (genome) into mRNA (transcriptome), which is then translated into proteins. Every step is regulated by processes, some of which are listed here above the relevant step in the flow of information from DNA into proteins. Engineered experiemental perturbations strategies mimic various natural regulatory steps, some of which are shown here. b) Protein activity is regulated intracellularly by a variety of processes, including its availability in a particular subcompartment, whether it is part of a complex, its tertiary structure/dynamics and its abundance. Each of these strategies are exploited experimentally for conditional post-translational control of protein function.

Fig 2

Subcellular localization as a strategy to control protein function. Upon translation in the cytoplasm, a protein's activity can be controlled by the availability of its substrates. Conditional dimerization systems (CID) have been used both to trigger protein activity by recruitment to its site of action, such as at the plasma membrane or in the nucleus, or to be inactivated by its aggregation or sequestration in a subcellular compartment where it cannot act.

Fig 3

Conditional dimerization rescues protein function. a) Proteins that require dimerization to function, eg. transcription factors, some enzymes, can be activated directly by hetero- or homodimerization b) Regulated protein fragment complementation. The protein's primary sequence is split into two pieces, each of which is fused to a dimerization domain. Dimerization allows reconstitution of the holo-protein. c) Split inteins. Self-excising intein protein domains can be fused with the split protein fragments to allow complementation of the protein of interest. Upon intermolecular excision and ligation of the intein, the extein fragments are fused to form a single active polypeptide with no intervening dimerization domains d) Expressed protein ligation. Similar to split inteins, except where one of the fragments is synthetically derived. This can be especially useful to produce functionalized proteins or proteins with homogenous stoichiometric post-translational modifications.

Fig 4

The ‘bump-hole’ strategy. The protein of interest is mutated to create a cavity so that it will accept both the natural substrate and a larger substrate analog. The substrate analog will only bind to the engineered mutant protein. The substrate analog can either be an inhibitor, to selectively inhibit the mutant protein, or it can add a novel functionality, so the enzyme's target is unnaturally modified, which can then be exploited for purification of its targets.

Fig 5

Allosteric control of protein function. a) Mutually exclusive folding. One protein domain is inserted into an exterior loop of another protein, creating mechanical stress on its structure. The inserted domain can become unfolded, relieving this stress, allowing the host protein to fold properly. Thermodynamically controlling the stability of each of the domains by the addition of ligands for either the host or inserted domain can control the relative stability of the two domains b) ‘Classic’ allostery. Binding at a site distal to the active site causes a conformational or dynamical change affecting the enzyme's active site. c) Light driven reversible intramolecular binding. Protein activity is sequestered by the close association of a Lov2 domain with the protein of interest. Flexibility introduced by unfolding of the J-alpha helix allows protein activity. Reversion is not controlled, but relies on dark-state relaxation d) Similar to c), except the photoconvertible protein dronpa allows direct reversibility sequential application by violet (400nm) or cyan (500nm) light.

Fig 6

Degradation mediated control of protein abundance. a) Schematic of the ubiquitin proteasome system (UPS). Normal proteins can be targeted for degradation by their regulated ubiquitylation or if they are damaged/misfolded. Addition of a minimum of four ubiquitin moieties target the protein to the proteasome for degradation. b) Each step in the UPS can be manipulated for regulated degradation of a protein of interest. i) directly targeting a protein to the proteasome obviates the need for ubiquitylation for degradation. ii) recruitment of an ubiquitin E3 ligase by the addition of a dimerizer causes ubiquitylation of the protein of interest substrate, followed by its subsequent targeting to the proteasome and degradation. iii) relying on natural ubiquitylation processes. Conditional regulation of natural ubiquitylation processes by triggering the N-end rule or recognition of destabilizing domains by protein quality control ubiquitin E3 ligases.