I'm trying to purify plant protein expressed in Ecoli. There are two mitotic kinases, the one I'm doing now is from Arabidopsis, inserted to pdest15 vector. I transformed E-coli C-43 pLysS by electroporation and selected transformants using ampicilin and chloramphenicol. I culture 1L of cells with antibiotics till OD is 0.6-0.8, then add IPTG to final concentration 1mM and continue culture o/n at 16C. The protein have low expression - that's why long induction time. Next I do sonication, centrifugation and use supernatant in AKTA with GSTrapFF column --> SDS-PAGE to check results. The column and AKTA protocol seem to be fine - I checked with other GST-protein, it worked. But with this one it fails. The most confusing thing is - somebody in the lab used the same protocol and it worked. I myself had used same protocol for another plant kinase - it also worked. I made new IPTG stock, it didn't help.
Any ideas, please, I'm lost