An exocellular esterase from the oil-degrading Acinetobacter venetianus RAG-1 was previously shown to enhance the emulsification and emulsion stabilization properties of the amphipathic, aminopolysaccharide bioemulsifier, emulsan [Bach H, Berdichevsky Y, Gutnick D (2003) An exocellular protein from the oil-degrading microbe Acinetobacter venetianus RAG-1 enhances the emulsifying activity of the polymeric bioemulsifier emulsan. Appl Environ Microbiol 69:2608-15]. This enhancement was specific for the RAG-1 esterase and was independent of catalytic activity. In this report, fragments from both the N'- and C'-termini were cloned as fusions to the C-terminus of the maltose-binding protein (MBP) and were tested for enhancement activity in the presence of the deproteinated form of emulsan, apoemulsan. The activity could be localized to the C-terminal third of the protein which exhibited the same activity as the intact enzyme. MBP itself was completely inactive and could be cleaved from the fusion without affecting the subsequent emulsification. However, the enhancement completely depended on the presence of a unique C-terminal 20 amino acid peptide not found in any other protein in the databases. In addition, progressive removal of amino acids from the N-terminus of the active MBP polypeptide resulted in a concomitant loss of activity, indicating that enhancement is also proportional to the size of the peptide fragment. The middle third and the C-terminal third of the enzyme each contained a copy of the conserved Cardin-Weintraub consensus sequence for protein binding to heparin. These sequences were not detected in homologous esterases from a closely related strain, Acinetobacter calcoaceticus BD413.