Dematin is an actin-binding protein abundant in red blood cells and other tissues. It contains a villin-type 'headpiece' F-actin-binding domain at its extreme C-terminus. The isolated dematin headpiece domain (DHP) undergoes a significant conformational change upon phosphorylation. The mutation of Ser74 to Glu closely mimics the phosphorylation of DHP. We investigated motions in the backbone of DHP and its mutant DHPS74E using several complementary NMR relaxation techniques: laboratory frame (15)N NMR relaxation, which is sensitive primarily to the ps-ns time scale, cross-correlated chemical shift modulation NMR relaxation detecting correlated mus-ms time scale motions of neighboring (13)C' and (15)N nuclei, and cross-correlated relaxation of two (15)N-(1)H dipole-dipole interactions detecting slow motions of backbone NH vectors in successive amino acid residues. The results indicate a reduction in mobility upon the mutation in several regions of the protein. The additional salt bridge formed in DHPS74E that links the N- and C-terminal subdomains is likely to be responsible for these changes.