[Chimera-users] Alteration of C-terminal AA-sequence within an X-ray model data set with Chimera

Tom Goddard goddard at sonic.net
Thu Apr 14 12:06:04 PDT 2011

Hi Heinrich,

   A few more comments about how to go about the fitting to address 
Elaine's problem 2 that your 30 residue tail might be moved by the 
fitting in the EM map into collision with the other parts of the 
protein.  To prevent that I would suggest subtracting from the EM map a 
simulated map representing the fit protein without including the 30 
C-terminal amino acids.  Then fit the 30-amino acid fragment into the 
difference map. You compute the simulated map with the Chimera "molmap" 
command, and to subtract that from the EM map you use "vop subtract" 
with the minRMS option set to true in order to scale the simulated map 
so that it has comparable density values as the experimental map.

molmap #1:123-400.A 12.5 model #2
vop subtract #0 #2 minRMS true

The first command makes the simulated map from a specific residue range 
and chain of your atomic model.  The simulated map is given the model id 
#2.  The second command subtracts this simulated map from the 
experimental EM map #0 and will create a new map that you can use for 




> Whoops, I just realized that "Fit in Map" also has an option to fit only the selected atoms.  Thus it is not necessary to do any deleting or combining as described for (B), which makes it much simpler.
> Sorry about that!  However, the problems (1) and (2) mentioned below still apply.
> Elaine
> On Apr 14, 2011, at 11:01 AM, Elaine Meng wrote:
>> Dear Heinrich,
>> I can think of a couple possibilities.  First you would open the structure and the density map.  Opening the density map will automatically start Volume Viewer, in which you can adjust the display type (surface, mesh, etc.) and contour level by moving the slider on the histogram.
>> <http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/volumeviewer/framevolumeviewer.html>
>> <http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/volumeviewer/volumeviewer.html#dispsurf>
>> Then...
>> (A) you could select those 30 residues, then use "Movement Mouse Mode" (under Tools... Movement) "move selection" option to manually reposition them.   However, that would not automatically fit the residues to density, it would just let you rotate and translate them as a rigid body yourself with the mouse, which can be difficult if you are trying to dock into density.  The selecting could be done in several ways, such as by command (e.g. "select #0:356-end.a" to select model 0 chain A residues 356 to the end) or interactively in the Sequence tool (under Favorites in the menu).  After the movement you could save the modified structure with "File... Save PDB" in the Chimera menu or the "write" command, and you would probably want to save "relative to" the map model.
>> <http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/movemode/movemode.html>
>> <http://www.cgl.ucsf.edu/chimera/docs/UsersGuide/savemodel.html#pdb>
>> (B) you could make those 30 residues a separate model and then automatically fit them as a rigid whole into the density using "Fit in Map" (under Tools... Volume Data).   You would delete the rest of the structure, for example with command "delete ~#0:356-end.a"  to delete all atoms EXCEPT model 0 residues 356-end in chain A.  Next, using the mouse, manually move it to an approximate position relative to the map.  You can freeze the map and move only the structure by un-checking the "Active" box for the map model below the Command Line or in the Model Panel (both tools are under Favorites in the menu).  Then use Fit in Map, which will perform a local optimization.  You may want to run it a few times to see if the result improves.
>> <http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/fitmaps/fitmaps.html>
>> Saving a whole model is less convenient than in (A).  You would need to open another copy of the structure, delete the opposite part (e.g. command "delete #0:356-end.a") and probably also fit that one into the density.  You could either save both as PDB relative to the map model and then manually combine them into one PDB file, or in Chimera merge the models (see command "combine" or the "copy/combine" function in the Model Panel), possibly add the bond if you need it (see command "bond" or the Build Structure tool) and then save the PDB of the combined model relative to the map model.
>> Even if method (B) is not too long for you, I can think of two problems:
>> (1) It is not flexible fitting.  Each part is treated as a rigid body, and you have to decide yourself how to split a structure into parts.
>> (2) The parts do not "see" each other, there is nothing to stop them from bumping into each other or even going to the same place in the density map.   In the Chimera daily builds, we have recently added a tool that addresses this problem:  MultiFit (under Tools... Volume Data) will perform simultaneous fitting of multiple rigid structures.  However, it currently assumes overall Cn (cyclic) symmetry, so it cannot be applied to your problem.   We hope to have it working for structures without cyclic symmetry soon, but that is not yet available.
>> <http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/multifit/multifit.html>
>> I hope this helps,
>> Elaine
>> ----------
>> Elaine C. Meng, Ph.D.
>> UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab
>> Department of Pharmaceutical Chemistry
>> University of California, San Francisco
>> On Apr 14, 2011, at 2:01 AM, Heinrich Lünsdorf wrote:
>>> Dear Elaine,
>>> here I'm again to ask for your constructive help on another item. My question: Is it possible with Chimera functions and commands to pick up and activate an AA-sequence, i.e. the last 30 C-terminal AAs, and to 'move'  or 'bend' them in parts or as a whole but without changing or moving the rest of the X-ray model to get the C-terminal AA-tail fit along with an overlaid EM-density model?? Can such alterations be stored for the original X-ray model?
>>> Thank you cordially for your help in advance.
>>> Sincerely yours,
>>> Heinrich
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