Pseudosubstrate sequence may not be critical for autoinhibition of smooth muscle myosin light chain kinase

It has been hypothesized that basic residues in the autoinhibitory region of myosin light chain (MLC) kinase, which resemble the substrate sequence, interact with the catalytic core via charge interaction and thus inhibit the kinase activity (pseudosubstrate inhibitory hypothesis). In the present study, we produced seven MLC kinase mutants in which the residues in the autoinhibitory region are deleted to various extents, and determined the residues crucial for the autoinhibition of the kinase activity. The activities of MT799 (1-799) and MT796 (1-796) were completely inhibited, whereas MT793 (1-793), MT791 (1-791), MT787 (1-787) and MT783 (1-783) were constitutively active. The tryptic proteolysis of MT799 and MT796 activated the kinase activity, presumably due to the removal of the residues essential for autoinhibition. The mutants which showed the constitutively active kinase activity were not further activated by tryptic proteolysis, suggesting that the residues crucial for autoinhibition were already deleted. On the other hand, MT795 (1-795) was partially constitutively active (33% of maximum activity) and the tryptic proteolysis further activated the enzyme activity, suggesting that MT795 loses part of the residues essential for autoinhibition. The substitution of the residues Tyr794-Met795 but not Lys793 of untruncated MLC kinase significantly increased the Ca2+/calmodulin-independent kinase activity. These results clearly show that the region Tyr794-Met795-Ala796 is critical for autoinhibition. This study shows that the pseudosubstrate sequence is not critical for the autoinhibition mechanism of MLC kinase.