Some concepts in science are counter-intuitive, like the Monty Hall problem or the Mpemba effect. Occasionally, this is also true for terminology, despite the best efforts of scientists to ensure that their work can be explained unambiguously to newcomers. Specifically, in our field of protein structure prediction, the word “decoy” has been used to mean one of many conformations generated by a de novo modelling protocol such as Rosetta, or alternative conformations of loops produced by an ab initio program e.g. Sphinx. Though slightly baffled by this usage when I started working in the field, I have now become so familiar with its strange new meaning that I have to remind myself to explain it in talks to a more general audience, or simply aim to avoid the term altogether. Nonetheless, following a heated discussion over the term in a recent group meeting, I thought it would be interesting to trace the roots of the new meaning.
Let’s begin with a definition from Google:
a person or thing used to mislead or lure someone into a trap.“we need a decoy to distract their attention”
So we start with the idea of something distracting, resembling the true thing but with the intent to deceive. So how has this sense of the word evolved into what we use now? I attempted to dig out the earliest mention of decoy for a computationally generated protein conformation with a Google scholar search for “decoy protein”, which led to the work of Thomas and Dill published in 1996. Here the authors describe a method of distinguishing the native fold of a protein from the sequence threaded, without gaps, onto alternative structures from the PDB. This problem of discrimination between native and non-native had been carried out previously, but Thomas and Dill chose to describe the alternatives as “decoy conformations” or just “decoys”.
A similar problem was commonly attempted over the following years, of separating native structures from sets of computationally generated conformations. Due to the demands of conformer generation at this time, some sets were published themselves in online databases to be used as a resource for training scoring functions.
When it comes to the problem of de novo protein structure prediction, unfortunately it isn’t as simple as picking out the correct answer from a population of incorrect answers. Even among hundreds of thousands of conformations generated by the best methods, the exact native crystal structure will not be found (though a complication here that the protein is dynamic and will occupy an ensemble of native conformations). Therefore, the aim of any scoring function in structure prediction is instead to select which incorrect conformation is closest to the native structure, hoping to obtain at least the correct fold.
It is for this reason that we move towards the idea of choosing a model from a pool of decoys. Zhu et al. (2003) use “decoy” in precisely this way:
“One strategy for ab initio protein structure prediction is to generate a large number of possible structures (decoys) and select the most fitting ones based on a scoring or free energy function”
This seems to be where the idea of a decoy as incorrect and distracting is lost, and takes on its new meaning as one of a large and diverse set of protein-like conformations, which has continued until now.
So is it ever helpful to refer to “decoys” as opposed to “models”? What is communicated by “decoy” that is not achieved by using the word “model”? I think this may come down to the impression which is given by talking about a pool of decoys. People would not generally assume that each decoy on its own has any effective use for prediction of function. There is a sense that this is not the final result of the structure prediction pipeline, there is work yet to be done in refining, clustering, and making human judgments on the suitability of the output. Only after these stages would I feel more comfortable using the word “model”, to express the greater confidence we have in the structure (small though that may be in the de novo structure prediction world). However, the inadequacy of “model” does not alone justify this tenuous usage of “decoy”. Perhaps we could speak more often about populations of “conformations”. In any case, “decoy” is widespread in the community, and easily understood by those who are most likely to be reading, reviewing and editing the literature so I think we will be stuck with it for a while yet.