Protein purification: iodoacetic acid -does it stuff up cysteines

[Orak]

I have been using iodoacetic acid (IAA) as a protease inhibitor for purifying
recombinant transcription factors.  However, someone recently told me
that IAA works by modifying cysteine residues.  Can someone confirm this
for me?  Exactly how does it work?
This could be a problem for me since ETS transcription factors are
reliant on spaced cyteines for DNA binding activity.

Thanks for any help

Ross