To build a model of a cyclic protein, you must do two things:
- Disable the default patching, which would create a C and N terminus.
- Use the LINK patching residue in the special_patches routine to link the last and first residues.
For example, the script below builds a cyclic model of 1fdx using 5fd1 as a template (attachment:alignment.ali is the example from the Modeller distribution).
/!\ It is important that you get the numbering of residue_ids correct; the residue numbers must correspond to the last and first residue in the cyclic protein respectively (in the model, not the template). If in doubt, build a regular model first and look at the residue numbers in the final models or `.ini` file.
#!python
from modeller.automodel import *
log.verbose()
env = environ()
# Disable default NTER and CTER patching
env.patch_default = False
class mymodel(automodel):
def special_patches(self, aln):
# Link between residues 54 and 1 to make chain cyclic:
self.patch(residue_type='LINK', residue_ids=('54', '1'))
# Use the new 'mymodel' class rather than 'automodel'
a = mymodel(env, alnfile='alignment.ali', knowns='5fd1', sequence='1fdx')
a.starting_model = a.ending_model = 1
a.make()