SPECIFICITY AND SYMMETRY IN THE INTERACTION OF CALMODULIN DOMAINS WITH THE SKELETAL-MUSCLE MYOSIN LIGHT-CHAIN KINASE TARGET SEQUENCE

The specificity of interaction of the isolated N- and C-terminal domai ns of calmodulin with peptide WFFp (Ac-KRRWKKNFIAVSAANRFK-amide) and v ariants of the target sequence of skeletal muscle myosin light chain k inase was investigated using CD and fluorescence. Titrations show that two molecules of either domain bind to 18-residue target peptides. Fo r WFFp, the C-domain binds with 4-fold higher affinity to the native c ompared with the non-native site; the N-domain shows similar affinity for either site. The selectivity of the C-domain suggests that it prom otes occupancy of the correct binding site for intact calmodulin on th e target sequence. Far UV CD spectra show the extra helicity induced i n forming the 2:1 C-domain-peptide or the 1:1:1 C-domain-N-domain-pept ide complex is similar to that induced by calmodulin itself; binding o f the C-domain to the Trp-4 site is essential for developing the full helicity. Calmodulin-MLCK-peptide complexes show an approximate two-fo ld rotational relationship between the two highly homologous domains, and the 2:1 C (or N)-domain-peptide complexes evidently have a similar rotational symmetry. This implies that a given domain can bind sequen ces with opposite peptide polarities, significantly increasing the pos sible range of conformations of calmodulin in its complexes, and exten ding the versatility and diversity of calmodulin-target interactions.