Samples of the proteins expressed in E.coli from the wildtype gene fucA1 and the mutants fucA1 A and fucA1 sm , purified as described, were fractionated on an SDS-PAGE. Protein bands were excised from the gel, washed in 50 mM ammonium bicarbonate, pH 8.0, in 50% acetonitrile, reduced with 10 mM DTT at 56 C for 45 min and alkylated with 55 mM iodoacetamide for 30 min at room temperature in the dark. The gel pieces were washed several times with the buffer, resuspended in 50 mM ammonium bicarbonate and incubated with 100 ng of trypsin for 2 h at 4 C and overnight at 37 C. The supernatant containing peptides was analysed by MALDIMS on an Applied Biosystem Voyager DE-PRO mass spectrometer using a-cyano-4-hydroxycynnamic acid as matrix. Mass calibration was performed by using the standard mixture provided by manufacturer.