I know that quaternary protein structures are formed exclusively via non-covalen

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?

Explanation / Answer

If I understand the question correctly, you're thinking something along the lines of modular tertiary domains, but in a quaternary sense?

For a reversible interaction, heterotrimeric G proteins spring to mind. Once activated by a G protein coupled receptor (GPCR), the alpha subunit and the beta-gamma complex dissociate from the receptor and from each other. They can both go on to affect signaling by binding to various effector proteins, depending on the particular pathway in which they are involved, and their activities can be mediated by other proteins (e.g. the alpha subunit GTPase regulated by GAP proteins).

I spoke to my Protein Engineering professor, and she also directed me to a few examples, although I'm not sure they're quite what you're looking for in the strictest sense.

It seems that pretty much any protein that interacts with two or more separate protein complexes could be considered an answer. Neither I nor my professor have managed to come up with an example of an irreversible assembly of common subunits, but she is of the opinion that, in all the different possible protein structures, there is probably at least one example of everything, no matter how outlandish it seems.

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