Monoclonal anti-FLAG antibodies react with a new isoform of rat Mg2+ dependent protein phosphatase beta.

The FLAG peptide has been widely used as a multi-purpose tag for the identification and detection of recombinant FLAG fusion proteins. The practicability of this approach depends on specific detection of FLAG fusion proteins with no or very little cross-reactivity to cellular proteins. We have isolated a rat cDNA clone coding for a new splicing isoform of Mg2+ dependent protein phosphatase beta (MPP beta) by screening a rat brain expression library with monoclonal antibody Anti-FLAG M2. MPP beta reacts strongly both as a MPP beta-beta-galactosidase- and as a glutathione S-transferase fusion protein with anti-FLAG M2 antibodies. Sequence analysis of MPP beta revealed a sequence motif with five out of eight amino acid residues identical to the FLAG peptide hitherto believed to be mono-specific.