Identification of transglutaminase-reactive residues in S100A11

The recent finding that S100A11 is a component of the keratinocyte cornified envelope (CE) (Robinson, N. A., Lapic, S., Welter, J. F., and Eckert, R. L. (1997) J. Biol. Chem. 272, 12035-12046) suggests that S100A11 is a transglutaminase (TG) substrate. In the present study we show that S100A11 forms multimers when cultured keratinocytes are challenged by increased levels of intracellular calcium and that multimer formation is inhibited by the TG inhibitor, cystamine. These S100A11 multimers appear to be incorporated into the CE, as immunoreactive S100A11 is detected in purified envelopes prepared from cultured cells and from foreskin epidermis. To study S100A11 as a transglutaminase substrate, recombinant human S100A11 (rhS100A11) was used in a cell-free cross-linking system. [14C]Putrescine, a primary amine, labels rhS100A11 in a TG-dependent manner. Trypsin digestion of [14C]putrescine-labeled rhS100A11 releases one radiolabeled peptide, Ala98-Lys103. The glutamine residue in this segment, Gln102, is the site of radiolabel incorporation indicating that Gln102 functions as an amine acceptor. The ability of S100A11 to form multimers indicates that it also has a reactive lysine residue that functions as an amine donor. To identify the reactive residue, we compared the high pressure liquid chromatography profile of trypsin-digested rhS100A11 monomer to that of cross-linked rhS100A11. A unique cross-linked peptide was purified and identified as Met-Ala-Lys3-Ilu-Ser-Ser-Pro-Thr-Glu-Thr-Glu-Arg cross-linked via an Lys3-Gln102 isopeptide bond to Ala-Val-Pro-Ser-Gln102-Lys. These studies show that S100A11 is post-translationally modified by transglutaminase, that it can be cross-linked to form multimers, that it is present in CEs from cultured keratinocytes and in vivo epidermis, and that Lys3 and Gln102 are specific sites of cross-link formation.