A functional MBP–YscG hybrid protein functions to stabilize YscF, but YscF is not secreted in the absence of YscE. Y. pestis KIM5-3001, isogenic ΔyscE, ΔyscG, ΔyscEG mutants, and mutants complemented with pMAL-c2 (pMBP), pMBP-YscE, pMBP-YscG and/or pFLAG-MAC-YscG-YscE-6xHis were grown in the presence (+) or absence (−) of 2.5 mM CaCl2 and the cell pellets and culture supernatants were analysed by SDS-PAGE and immunoblot analysis with antibodies specific for YopN, YscF, MBP, penta-His and the FLAG epitope. (a) Expression of MBP-YscE in ΔyscE Y. pestis restores YscF expression and YopN secretion. (b) Expression of MBP-YscG in ΔyscG Y. pestis restores YscF expression and YopN secretion. (c) Expression of MBP-YscG but not MBP or MBP-YscE in ΔyscE ΔyscG Y. pestis restores stable YscF expression but not YscF secretion. The location of full-length YscF is shown by an arrowhead.