a lot of halophilic protein is unfolded in the absence of high concentration of salt where most proteins favors the folded state. I wonder if such halophilic protein can be considered as IDPs in the absence of salt?
Dear Xinji Zhu ,
It is known that a high negative surface charge density is a characteristic feature of halophilic proteins, because of this these type of proteins can remain soluble in high salt. The pI of the halophile proteome is unique in having an exclusively acidic distribution with a median value of ∼4.5. Soluble at high salt often means highly unfolded.
However intrinsically disordered (at high salt) does not mean without folding features and capabilities, a fairly recent report indicates foldability properties of a halophilic type of protein:
https://www.pnas.org/content/110/6/2135.short
and postulates a halophile origin of foldable proteins:
https://www.frontiersin.org/articles/10.3389/fmicb.2013.00418/full
The picture that is emerging when it comes to intrinsically disordered proteins is the enormous structural heterogeneity of IDPs with a continuous spectrum of protein structures:
https://onlinelibrary.wiley.com/doi/pdf/10.1002/pro.2261
So the answer might be yes in the presence of high salts, but with a big realisation that this is not the whole story. Abscence of salts, presence of binding partners and so on induce all sorts of folding capabilities but this class of proteins initiated the realisation that the structure-function hypothesis is even more complex then anticipated before.
Hope this will help you a bit further.
Dear Xinji,
Rob hit the problem on the head. I wanted only to add a clarifying remark. Every protein represents the whole continuum of folded to unfolded states. The so called folding principle by definition cannot cover all the space because the dynamical coupling of proteins with the solvent define their function. In another words every protein is slightly unfolded all the time, and only external conditions as well as its evolutionary purpose defines how mobile versus how folded it is in the particular set of conditions. So using the name "intrinsically disordered" is only a marketing slogan as it is a name "halophilic protein".
Good Luck.
Bog
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