Levansucrase from Acetobacter diazotrophicus SRT4 is secreted via periplasm by a signal-peptide-dependent pathway

Acetobacter diazotrophicus SRT4 secretes a constitutive levansucrase (LsdA) (EC 2.4.1.10) that is responsible for sucrose utilization. Immunogold elec tron microscopical studies revealed that LsdA accumulates in the periplasm before secretion. The periplasmic and extracellular forms of the enzyme wer e purified to homogeneity. Both proteins exhibited similar physical and bio chemical characteristics indicating that LsdA adopts its final conformation in the periplasm. The N-terminal sequence of mature LsdA was Glu-Gly-Asn-P he-Ser-Arg as determined by PSD-MALDI-TOFMS (post-source decay-matrix-assis ted laser desorption/ionization-time-of-flight mass spectrometry). Comparis on of this sequence with the predicted precursor protein revealed the cleav age of a 30-residue typical signal peptide followed by the formation of the pyroglutamic acid (pGlu) residue. Thus, in contrast with other Gram-negati ve bacteria, A. diazotrophicus secretes levansucrase by a signal-peptide-de pendent mechanism.