Calcium coordination studies of the metastatic Mts1 protein

Mts1, also known as S100A4, is an 11 kDa calcium-binding protein strongly linked to metastasis. As a member of the S100 protein family, Mts1 is predicted to contain four alpha-helices and two calcium-binding loops, the second of which forms a canonical EF hand, while the first is a pseudo-EF hand, using two extra residues and principally backbone carbonyls rather than side chain oxygens to coordinate calcium. Here we follow chemical shift changes which occur in Mts1 upon titration of calcium. The results are consistent with calcium coordination by the EF hands described above. Filling of the first (pseudo) EF hand occurs at a lower calcium concentration than does filling of the second (canonical) EF hand. Concurrent with filling of site I, resonances from much of helix 4 vanish while the chemical shifts of a possibly nascent helical segment immediately C-terminal to helix 4 increase in helical character. Other smaller changes are seen, including a change in the linker joining helix 2 and helix 3. Since binding of effector molecules to S100 proteins has been shown to involve the C-terminus and linker regions, these calcium-induced changes have implications for the role of Mts1 in metastasis.