Expression of recombinant rat myo-inositol 1,4,5-trisphosphate 3-kinase B suggests a regulatory role for its N-terminus

We have expressed rat myo-inositol 1,4,5-trisphosphate (IP₃) 3-kinase B as both a full-length, recombinant, non-fusion protein and a full-length, recombinant, fusion protein with maltose-binding protein (MBP) in Escherichia coli. The fusion protein with MBP is soluble, binds calmodulin and is enzymically active whereas the non-fusion protein is insoluble and does not bind calmodulin unless co-expressed with bacterial chaperone proteins (either GroES and GroEL, or DnaK, DnaJ and GrpE). However, soluble, calmodulin-binding non-fusion IP₃ 3-kinase B is enzymically inactive. The catalytic domain of the enzyme has previously been shown to reside near the C-terminus; the results we present suggest an auto-regulatory role for the N-terminus.