how to purify a protein with PI=4.8

[Dr Engelbert Buxbaum]

xuyihui wrote:
> 
> now we have a fusion protein expressed in Ecoli GI 724,it has a PI4.8 and
> 40KD.between the proteins we have a POLY_his,but the usual method for
> his-tag proteins is not efficient for it .and I use the DEAE chromotography, > it is not efficient yet,How can I do?

A pI of 4.8 means that the protein will be positively
charged (net) above pH 4.8, where most of the other proteins
will bear net negative charges. This is a rather low value
compared with most proteins and can be used for purification
in two ways (assuming that the protein is stable at
moderately low pH):

1) A strong cation exchanger (i.e. sulfopropyl-groups) at
about pH 5.5 should bind your protein, but not much else. 

2) On the other hand, an anion exchanger should at this pH
bind almost all other proteins, but not yours. 

The eluate from this purification could then be passed onto
a gel chromatography column, for further purification (by
molecular weight) and possibly buffer exchange to a more
physiological pH. 

Other methods like AS- or PEG-precipitation or HIC may also
work, that that you would hve to find out yourself.