BiologicalUnit

Note: PyMOL 1.8 can load biological units from mmCIF files with the assembly setting.

This script can be used to re-create biological units for proteins. (This was created as a workaround of PyMOL's semi-functioning Symexp command.) It's also a fun script to play with for learning about symmetry.

• 

  single unit, before running this program

• 

  after the expansion of the 60 units

• 

  Example creating the biological unit for PDB 1RMV created from fiber diffraction.

Usage

load /path/to/some/pdbFile.pdb
symMat = readSymmetry("/path/to/some/pdbFile.pdb","pdbFile")
biologicalUnit("mates", "pdbFile", symMat)

The Code

#
# Jason Vertrees <Jason-dot-Vertrees-at-schrodinger_dot_com>, 2010.
#
import pymol
from pymol import cmd

def readSymmetry(inFile, verbose=None):
  """
  This function will read "inFile" and glean the
  symmetry operations, if any, from it.
  
  PARAMS
    inFile
      (string) path to PDB file
      
    verbose
      (boolean) if verbose is not None, print more
      
  RETURNS
    matrix
      Array of lists.  One 16-element list per symmetry operation.  Feed this matrix
      into manualSymExp in order to make the other symmetry mates in the biological unit
  """
  # a remark-350 lines has:
  # REMARK 350 BIOMTn TAGn X Y Z Tx
  REM, TAG, BIOMT, OPNO, X, Y, Z, TX = range(8)
  
  thePDB = open(inFile, 'rb').readlines()
  
  matrices = []
  curTrans = -1
  
  # The transformation is,
  # output = U*input + Tx
  
  for l in thePDB:
    tokens = l.split()
    if len(tokens)!=8:
      continue
    if tokens[REM]=="REMARK" and tokens[TAG]=="350" and tokens[BIOMT].startswith("BIOMT"):
      if tokens[OPNO]!=curTrans:
        # new transformation matrix
        matrices.append([])
      
      matrices[-1].append( map( lambda s: float(s), tokens[X:]))
      curTrans = tokens[OPNO]

  if verbose!=None:
    print "Found %s symmetry operators in %s." % (len(matrices), inFile)
  return matrices


def biologicalUnit(prefix, objSel, matrices ):
  """
  Manually expands the object in "objSel" by the symmetry operations provided in "matrices" and
  prefixes the new objects with "prefix".
  
  PARAMS
    prefix
      (string) prefix name for new objects
    
    objSel
      (string) name of object to expand
      
    matrices
      (list of 16-element lists) array of matrices from readSymmetry
      
    RETUNRS
      None
  
    SIDE EFFECTS
      Creates N new obects each rotated and translated according to the symmetry operators, where N
      equals len(matrices).
  """
  for m in matrices:
    n = cmd.get_unused_name(prefix)
    cmd.create(n, objSel)
    s1 = "%s + (x*%s + y*%s + z*%s)" % (m[0][3], m[0][0], m[0][1], m[0][2])
    s2 = "%s + (x*%s + y*%s + z*%s)" % (m[1][3], m[1][0], m[1][1], m[1][2])
    s3 = "%s + (x*%s + y*%s + z*%s)" % (m[2][3], m[2][0], m[2][1], m[2][2])
    cmd.alter_state(1, n, "(x,y,z) = (%s, %s, %s)" % (s1, s2, s3) )

Notes

This is slow compared to Symexp; use the above for learning, playing and when Symexp doesn't work as advertised.

See Also

• BiologicalUnit/Quat (alternative implementation)
• Symexp
• SuperSym
• PDB Tutorial Biol. Units
• Wikipedia article
• assembly