Protein dimers on SDS gels -- Can they be non-covalent?

[Toshiya Endo]

In article <35lvvh$5q2 at news.doit.wisc.edu>, mcmahan at psl.wisc.edu writes:
> The second peak was mostly the right size with an additional
> band that also cross reacted with the monoclonal.  It size is roughly that of
> a dimer.  The dimerization is resistant to 0.1 M DTT so is probably not a
> disulphide.  Does anyone know of other associations that could be going 
on?
> Is it possible that a non-covalent association could be resistant to SDS?

Streptavidin forms a tetramer through non-covalent interactions.  The 
streptavidin tetramer is resistant to dissociation even in the presense of SDS 
plus heat treatment at 60C (20 min); it dissociates only after 5min of boiling 
(Meth. Enzymology vol 184, p87).  We tested several streptavidin fusion 
proteins, which were synthesized in a cell-free translation system in the 
presence of biotin, and found that some of them are hard to dissociate even 
after incubation at 95C for 5 - 10 min in the presence of S