disulphide bonds in nuclear proteins

Frank Fuerst ffrank at rz.uni-potsdam.de
Mon Feb 19 10:03:04 EST 2001

Lyndsay_Smith at xenova.co.uk (Lyndsay Smith) wrote:

>I'm interested in some information on disulphide bond formation in proteins.
>I've read that disulphide bonds are more stable in extracellular proteins
>than in cytosolic proteins. 

I doubt this, or at least I would put it otherwise. The cellular
environment is quite reducing in the cytosol, while it is more
oxidizing in the ER, periplasm etc. Thus, cytosolic proteins usually
don't have any disulfide bridges in their native structure, whereas
secreted proteins often have.

Of course, if someone found (or has found, don't know) a cytosolic
protein that indeed has disulfide bonds, then it would be clear that
this protein is much more stable in its oxidized than in its reduced
form, and this stabilization is bigger than with other proteins. So
you can say that this cytosolic disulfide bond is more stable than
extracellular ones just because, to form at all, it _has_to_ be
stabilized by its structural environment more than is needed for
extracellular proteins.

>My query is are disulphide bonds stable in
>nuclear proteins? 

I don't know, but it should be possible to look up the redox state of
the nucleus if you know the right book.

Unfortunately, recent investigations have shown that the
GSH/GSSG-ratio (glutathion), which is usually used to quantify if a
cellular compartment is reducing or oxidizing, is not the redox agent
acting on proteins. In eucaryotes, disulfide bond formation is
mediated by PDI in the ER. Tu et al. (Science 290 (2000), p. 1571-74)
have shown that PDI is re-oxidized by the protein Ero1p (in yeast)
which in turn is re-oxidized by soluble FAD. It seems that the
FAD_red/FAD_ox-ratio is kinetically decoupled from the GSH/GSSG-ratio,
so the latter is not a good measure when talking about eucaryotic
disulfide bond formation.

Regards, Frank

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