S100A6 and S100A11 are specific targets of the calcium- and zinc-binding S100B protein in vivo

In solution, S100B protein is a noncovalent homodimer composed of two subun its associated in an antiparallel manner. Upon calcium binding, the conform ation of S100B changes dramatically, leading to the exposure of hydrophobic residues at the surface of S100B. The residues in the C-terminal domain of S100B encompassing Phe(87) and Phe(88) have been implicated in interaction with target proteins. In this study, we used two-hybrid technology to iden tify specific S100B target proteins. Using S100B as bait, we identify S100A 6 and S100A11 as specific targets for S100B. S100A1, the closest homologue of S100B, is capable of interaction with S100B but does not interact with S 100A6 or S100A11. S100B, S100A6, and S100A11 isoforms are co-regulated and, co-localized in astrocytoma U373 cells. Furthermore, co-immunoprecipitation experiments demonstrated that Ca2+/Zn2+ stabilizes S100B-S100A6 and S100B- S100A11 heterocomplexes. Deletion of the C-terminal domain or mutation of P he(87) and Phe(88) residues has-no effect on S100B homodimerization and het erodimerization with S100A1 but drastically decreases interaction between S 100B and S100A6 or S100A11. Our data, suggest that the interaction between S100B and S100A6 or S100A11 should not be viewed as a typical S100 heterodi merization but rather as a model of interaction between S100B and target pr oteins.