To test whether fucA1 is expressed in S.solfataricus we analysed the extracts of cells grown on yeast extract, sucrose and casaminoacids medium (YSM). Accurate assays showed that S.solfataricus extracts contained 3.4 · 10 À4 units mg À1 of a-fucosidase activity. These very low amounts hampered the purification of the enzyme. The extracts of S.solfataricus cells grown on YSM revealed by western blot a band of a molecular mass >97 kDa and no signals were detected with the pre-immune serum confirming the specificity of the anti-Ssa-fuc antibodies ( Figure 5A) . The different molecular mass may result from post-translational modifications occurred in the archaeon or from the incomplete denaturation of a protein complex. In particular, the latter event is not unusual among enzymes from hyperthermophilic archaea (31, 32) . To test which hypotheses were appropriate, cellular extracts of S.solfataricus were analysed by western blot extending the incubation at 100 C to 2 h. Interestingly, this treatment shifted the high-molecular mass band to 67.6 ± 1.2 kDa ( Figure 5B and C) , which still differs from that of the recombinant Ssa-fuc, 58.9 ± 1.2 kDa, leaving the question on the origin of this difference unsolved. To try to shed some light we immunoprecipitated extracts of S.solfataricus with anti-Ssa-fuc antibodies and we analysed the major protein band by MALDIMS. Unfortunately, we could not observe any peptide compatible with the fucosidase because the heavy IgG chain co-migrated with the band of the expected molecular weight (data not shown).