Consider a signaling molecule, say a member of a MAP kinase cascade (picture at right). A given protein can exist in either an inactive (A) or active (B) form. When active, the kinase phosphorylates some downstream target, otherwise it just sits in your cytoplasm taking a nap. Typically in this kind of system the active form of the kinase is also the phosphorylated form (red B). It’s typical to say something like, “The kinase is activated by phosphorylation.” At the same time we know from some of Dorothee’s work with Dave Wemmer that certain bacterial proteins that get phosphorylated already sample their active conformations even before they are modified (blue B).
Even for systems where this kind of sampling hasn’t been directly demonstrated it’s reasonable to assume it takes place. After all, the active and inactive structures have the same amino acids to work with. Unless the phosphate group itself is a lynchpin of the new structure (perhaps by bridging two structural elements), then the active structure must be one the unmodified kinase can adopt. Naturally, we expect this structure to be higher in energy than the inactive state (so blue B is higher on our energy diagram than blue A), and that phosphorylation decreases the energy of the active state so that it is subsequently preferred (so red B is lower than blue A).
The implication of this is that, unless the unmodified active structure is much higher in energy than the inactive structure, some proportion of our kinase is active even when not phosphorylated. Perhaps this is as low as 1-2%, a fraction that’s difficult to detect directly. Still, because enzymes are so efficient, this quantity may be significant. Or, for a single protein, we could say that it adopts an active form without phosphorylation 1-2% of the time. But we tend to talk about phosphorylation and other post-translational modifications as if they were switches, with phrases like “protein X is turned on by phosphorylation”. The reality, though, is that the switch is less a matter of turning a protein on than of turning it on more.
This points to a reality far less clean and orderly than typically depicted in block schematics. Inappropriately active (i.e. active without modification) members of the various protein ensembles must give rise to a considerable amount of noise in biological information processes. The system must therefore have some way to distinguish the signal from the noise that’s more than just the binary on/off typically depicted and discussed. These filters could take several forms — for instance, the kinase of our kinase may mediate the interaction between our kinase and its target, though in this case inappropriate activation of the MAPKK could still give rise to signaling noise. Alternately, the phosphate could mediate the kinase – target interaction. Or the cell could simply have an inefficient signaling system, so that multiple nearly-simultaneous signaling events are necessary to activate a response.
Is this point important? Maybe and maybe not. Most of our experiments can only access the behavior of ensembles, so the ensemble nature of protein action is not likely to lead us astray. But as single-molecule studies become more popular it may be important to keep the ensemble perspective in mind so as not to be confused by their results. Moreover, a conceptually accurate picture of cellular signaling and regulation will require us to keep this feature in mind.