pI=9.5 protein will not bind to cation exchanger at pH 6.5

[Michael Kertesz]

In article <2q6job$21t at news.iastate.edu>, bipin at iastate.edu (Bipin K Dalmia) says:
>
>i'm trying to purify a protein expressed in e. coli. there is tons of it
>in the soluble extract. the pI is about 9.5. so naturally i tried using
>a cation exchanger at pH 6.5 but my protein flows right thru it. i've

>
How did you determine the pI? From the nucleic acid sequence? The isoelectric point in the 
native protein can differ by up to three or four pH units from this!! Check the pI on a native ief gel (if its important - otherwise just use a different system that does
separate the protein. There are lots of possibilities!)

Michael Kertesz

ETH - Institute of Microbiology
Zuerich.