Mass spectrum (MS2) of the isopeptide cross-link found in Muc2 from WT animals, but undetected in theTgm3-/,n= 3. Gln2928 and Lys2891 of mouse Muc2 are the acyl-donor and acceptor sites, respectively. These amino acids correspond to the ones in humans as indicated inFigure 2C. B-ions are marked in red and y-ions are marked in blue. The parent ion showed anm/z[M+2H]2+of 887.96. Cleaved by trypsin/AspN.
