I have been trying to express a rat protein in bacteria. The MBP-fusion expressed at very high level (~ 40 mg/L) while the GST-fusion and His-tag only gave inclusion bodies. The problem is that all protein runs in the void volume on a size-exclusion column (s-200, hepes, pH 7.4, 200 mM NaCl), no matter it is the intact MBP-fusion or cleaved sample. There is no Cys on this protein so there is unlikely any disulfide bond related problem. Anything I can do before I throw away this construct and try insect or mammalian cells? Thanks.