The Modeller Comparative modeling tool is an interface to the Modeller program, either installed locally or run on a web service hosted by the UCSF RBVI. The command-line implementation is modeller comparative. See also: Model Loops, Rotamers, Build Structure, AlphaFold, esmfold
Comparative modeling (sometimes called “homology modeling”) generates theoretical models of a protein using one or more known related structures and a sequence alignment of the known and unknown structures. The protein chain to be modeled is the target, and a known atomic structure used for modeling is a template.
Modeller is developed by the Šali Lab, and its users should cite:
Comparative protein modelling by satisfaction of spatial restraints. Šali A, Blundell TL. J Mol Biol. 1993 Dec 5;234(3):779-815.
Modeller comparative modeling requires the following for each target chain:
Modeling a multimer requires a multimeric template structure with the same stoichiometry and expected spatial relationship. Modeller uses this stoichiometry and spatial relationship rather than trying to calculate it.
The target sequence can be fetched from UniProt or opened from a file. That single sequence suffices as the “alignment” if the template is similar enough in sequence to associate with it (see an example). Otherwise, it can be used as a query to search the PDB for possible template structures using Blast Protein (which can be started from the Sequence Viewer context menu). Alternatively, a multiple sequence alignment of the target and template(s) can simply be opened from a file, if available.
Modeling a heteromultimer requires a separate sequence alignment for each unique chain. For example, modeling an α2β2 tetramer requires a template structure that is also a α2β2 tetramer with its two α subunits associated with one alignment containing the target α sequence, and its two β subunits associated with another alignment containing the target β sequence.
The Modeller Comparative tool can be opened from the Sequence or Structure Prediction section of the Tools menu and manipulated like other panels (more...). It is also implemented as the modeller comparative command.
One sequence alignment per target should be chosen (highlighted) in the top section of the dialog. Immediately below, for each chosen alignment, the name of the target sequence should be selected from the pull-down menu.
Modeller Comparative settings including the license key are automatically saved in the preferences. Other than the license key, the remembered settings apply only to the GUI; the command defaults are not changed.
Clicking Cancel simply dismisses the dialog, whereas OK sends information to the web service and initiates the calculation.
When results are returned, the new models are opened, listed in the Model Panel along with any other models present in ChimeraX, and automatically superimposed on the lowest-model-ID template structure (for comparative modeling) or the original structure (for loop modeling and refinement) with matchmaker. Scores for the models are shown in a Modeller Results panel:
Clicking a row in the panel shows the corresponding model and hides the others. Clicking elsewhere in the panel shows all of the models at once. Clicking a column header sorts on the values in that column.
The panel's context menu includes an option to Fetch Additional Scores using the SaliLab Model Evaluation Server:
The same Modeller license key as needed for comparative or loop modeling is also required by the evaluation server, but it does not need to be specified again. Fetching additional scores is also implemented as the command modeller scores.
After additional scores have been fetched, the context menu option changes to Refresh Scores to allow re-evaluation after models have been modified (for example, to delete untemplated regions built as extended tails). However, since re-evaluation does not use all of the same information as does scoring during the original modeling process, it may worsen pre-existing GA341 and zDOPE scores and should only be used after making modifications that are expected to improve the scores.