  We consider multi-chain protein native structures and propose a criterion
that determines whether two chains in the system are entangled or not. The
criterion is based on the behavior observed by pulling at both temini of each
chain simultaneously in the two chains. We have identified about 900 entangled
systems in the Protein Data Bank and provided a more detailed analysis for
several of them. We argue that entanglement enhances the thermodynamic
stability of the system but it may have other functions: burying the
hydrophobic residues at the interface, and increasing the DNA or RNA binding
area. We also study the folding and stretching properties of the knotted
dimeric proteins MJ0366, YibK and bacteriophytochrome. These proteins have been
studied theoretically in their monomeric versions so far. The dimers are seen
to separate on stretching through the tensile mechanism and the characteristic
unraveling force depends on the pulling direction.
