Are quaternary protein monomers unique to a particular protein complex?

Question

I know that quaternary protein structures are formed exclusively via non-covalent bonds. My biochemistry professor discussed a viral capsid that is essentially one quaternary structure with 240 individual monomers of 4 kinds. I suspect that for this protein coat to be stable, the non-covalent bonds holding it together must be rather **stable**. He also mentioned that different monomers can join to form the functional protein complex from relatively far distances, like across an entire cell, which once again leads me to believe that both monomers must be interacting with each other **very strongly**.My question is: Since a strong non-covalent interaction is required between (at least) two protein monomers to form the functional quaternary structure, can one particular protein be a monomer to an array of different quaternary complexes? Are there any examples of this?

Answer 1

Potential Protein Biochemist:

Try not to confuse stability of an individual non-covalently bonded site with the stability of an entire complex. The individual attractive sites, whether they are hydrogen bonded, hydrophobic-hydrophobic, or induced dipole, are fairly weakly attracted, but the cooperative effect of having larger areas bonded at many places, can result in a fairly strong attraction, in the sense that considerable energy would need to be supplied to break ALL the small bonds. What's neat, there, is that quarternary structure can drastically affect the enzymatic (etc.) action of the units (e.g., hemoglobin). For the second part of your question, it's likely that certain capsid proteins (in particular) might be partially conserved between closely-related viruses, but recall that viruses, too, need to be able to evolve their core proteins to counter their host's evolving defenses. So why not their capsid proteins, too?