Answer:


1ppt is a dimer. Packed on the hydrophobic patches, I assume. The students can read this in the PDB file header under REMARK 350: 

REMARK 350 COORDINATES FOR A COMPLETE MULTIMER REPRESENTING THE KNOWN
REMARK 350 BIOLOGICALLY SIGNIFICANT OLIGOMERIZATION STATE OF THE
REMARK 350 MOLECULE CAN BE GENERATED BY APPLYING BIOMT TRANSFORMATIONS
REMARK 350 GIVEN BELOW.  BOTH NON-CRYSTALLOGRAPHIC AND
REMARK 350 CRYSTALLOGRAPHIC OPERATIONS ARE GIVEN.
REMARK 350
REMARK 350 BIOMOLECULE: 1
REMARK 350 AUTHOR DETERMINED BIOLOGICAL UNIT: DIMERIC
REMARK 350 APPLY THE FOLLOWING TO CHAINS: A
REMARK 350   BIOMT1   1  1.000000  0.000000  0.000000        0.00000
REMARK 350   BIOMT2   1  0.000000  1.000000  0.000000        0.00000
REMARK 350   BIOMT3   1  0.000000  0.000000  1.000000        0.00000
REMARK 350   BIOMT1   2 -1.000000  0.000000  0.000000        0.00000
REMARK 350   BIOMT2   2  0.000000  1.000000  0.000000        0.00000
REMARK 350   BIOMT3   2  0.000000  0.000000 -1.000000        0.00000

Which means that there is a crystallographic two-fold axis that relates the one monomer that is given in the PDB file to its 2-fold mate that is not given in the file.

These matrices are called BIOMT matrices (and this is not going to be an exam question...). The first relates the molecule to itself, and the second to its 2-fold mate.