Regulation of Limulus skeletal muscle contraction.
Skeletal muscle contraction of Limulus polyphemus, the horseshoe crab, seemed to be regulated in a dual manner, namely Ca2+ binding to the troponin complex as well phosphorylation of the myosin light chains (MLC) by a Ca2+/calmodulin-dependent myosin light chain kinase. We investigated muscle contraction in Limulus skinned fibers in the presence of Ca2+ and of Ca2+/calmodulin to find out which of the two mechanisms prevails in Limulus skeletal muscle contraction. Although skinned fibers revealed high basal MLC mono- and biphosphorylation levels (0.48 mol phosphate/mol 31 kDa MLC; 0.52 mol phosphate/mol 21 kDa MLC), the muscle fibers were fully relaxed at pCa 8. Upon C2+ or Ca2+/calmodulin activation, the fibers developed force (357+/-78.7 mN/mm2; 338+/-69.7 mN/mm2, respectively) while the MLC phosphorylation remained essentially unchanged. We conclude that Ca2+ activation is the dominant regulatory mechanism in Limulus skeletal muscle contraction.
Ritter
O
O
Max Delbrück Center for Molecular Medicine, Berlin, Germany.
Haase
H
H
Morano
I
I
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002147:Calmodulin / Q000378:metabolism; D006737:Horseshoe Crabs; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000378:metabolism* / Q000502:physiology*; D018994:Myosin Light Chains / Q000378:metabolism*; D010766:Phosphorylation; D020033:Protein Isoforms / Q000378:metabolism
D002147:Calmodulin; D018994:Myosin Light Chains; D020033:Protein Isoforms; D002118:Calcium