Model Loops 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 loops. See also: Modeller Comparative, Rotamers, Build Structure, AlphaFold
Model Loops builds parts of a protein without using a template. Missing segments can be built de novo, or existing segments refined by generating additional possible conformations. Parts that need building or refinement are often loop regions. The protein sequence to be modeled is the target.
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.
The following are required for each chain to be modeled:
The target sequence can be taken from the structure metadata (and shown for a specific chain using the menu: Tools... Sequence... Show Sequence Viewer), or fetched from UniProt, or opened from a file. The sequence must be the same as the sequence of the structure, except that differences are permitted within the segments to be filled in or refined, as well as any adjacent positions designated as flexible; the residue types shown in the Sequence Viewer will be used to model those segments. Accuracy can be quite high for shorter loops, but falls off rapidly with length (>50% poor predictions for segments with 10+ residues including the flexible positions, see Table 1 in Fiser et al., Protein Sci. 9(9):1753 (2000)).
Modeling multiple different chains in a complex requires a separate sequence alignment for each unique chain. For example, simultaneously modeling the α and β chains within an α2β2 tetramer requires both α subunits to be associated with one alignment containing the target α sequence, and both β subunits to be associated with another alignment containing the target β sequence. To instead model only one αβ dimer within the tetramer, the other two structure chains should be dissociated from the sequences.
Model Loops 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 loops 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.
Model Loops 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.
The results dialog is the same as described for Modeller Comparative.