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methionyl aminopeptidase 1 (Escherichia coli) Uniprot accesssion P07906 EC-Number 3.4.11.18 http://merops.sanger.ac.uk/cgi-bin/famsum?family=M24 On Tuesday 26 September 2006 08:47 am, Yadong Yu wrote: > Hi, > > This is a ccp4 irrelevant problem but it may be interesting to this > group. > > Recently I sent 2 freshly purified protein samples for MALDI mass > spectra to determine their molecular weights. The results of these 2 > protein both were ~135 Da lower than their theoretical molecular > weights. It is certainly out of the range of normal error as my > proteins have MW less than 20 kDa. The accuracy of these MALDI > experiments is reliable, as evidenced by the exact 2-fold ratios > between the single-charge value and double-charge value. The > expression vector of these 2 proteins were sequenced. They were > expressed in Rosetta(DE3), an E.coli strain, and purified in native > conditions. Taken account of the size of this difference, it would > not be cleavage of some small group, e.g. amino group of Gln/Asn, > from certain residues, the more possible reason would be deprivation > of amino acid at either end of the protein. In this secenario, the > initial methionine may be lost because its peptide fragment (-H2O), > of 131 Da in size, matches the difference very well. > > Anyone experienced such problem? Is the head of protein already cut > during expression, or just lost in MALDI experiments? > > Yadong Yu > LCMN/NICHD/NIH > Bldg 35, Room 3B 1006 > 35 Lincoln Drive > Bethesda MD 20892 > (301)496-9347 > Fax (301)496-2396 > [EMAIL PROTECTED] -- Ethan A Merritt Biomolecular Structure Center University of Washington, Seattle WA
