Why does this protein seem to have multiple chains?
Some parts a the protein are a bit more flexible, also in the protein crystal. These parts don't give clear electron density after x-ray diffraction, and are removed from the resulting model structure. Often the N- and C-terminal ends are removed, this is something you'll find in many .pdb structures, but sometimes also loops are removed, resulting in two extra loose ends. 

The authors were nice to note in the comments section of the pdb file exactly which residues were removed from the model (#465).

There might be tools available to model loops into the structure so you'll have one continuous chain in the .pdb file, but I really don't know which one is best (of course they're all completely unreliable).