Protein unavailable for IMAC binding

[Bean Long]

Hi all,

I'm expressing several proteins in BL21(DE3) using 0.5 mM IPTG for 
induction. One protein is proving a little difficult in that it is 
successfully extracted into the soluble fraction but only a relatively 
small proportion of this is subsequently bound to IMAC. I say relatively 
small in that a huge amount does not bind to IMAC but recovery of pure 
protein is still quite good. It would seem that a large amount of the 
protein is forming soluble, yet unbindable (occluded tag), aggregates. I 
wondered if using lower IPTG concentrations would reduce the total 
protein concentration and therefore the likelihood of aggregation or is 
there another approach? I do not wish to use denaturing agents (e.g. 
urea or guanidine) during purification as the protein is bound for 
crystallisation. Any ideas?

Cheers,

Ben