The fusion of a short synthetic propeptide between the SP and the mature moiety is another innovative biotechnological tool to enhance protein secretion. One such propeptide (composed of nine amino acid residues, LEISSTCDA) was developed and was shown to enhance the SE of several heterologous proteins in L. lactis: NucB, NucT, (Table 3 ) [18] , the B. abortus L7/L12 antigen (Table  3 ) [19] , and the α-amylase of Geobacillus stearothermophilus (Table 3 ) [18] . Directed mutagenesis experiments demonstrated that the positive effect of LEISSTCDA on protein secretion was due to the insertion of negatively charged residues in the N-terminus of the mature moiety [25] . Furthermore, the enhancement effect does not depend on the nature of the SP, since the secretion of NucB fused to either SP Nuc or SP Usp45 was enhanced by LEISSTCDA insertion [25] . Strikingly, the enhancement of SE was reproducibly accompanied by an overall increase of protein yields as determined in Western blot experiments. This observation suggests that heterologous precursors are degraded by intracellular proteases when they are not efficiently secreted and that a higher secretion could be a way to escape proteolysis.