title: Smooth muscle calponin-caltropin interaction: effect on biological activity and stability of calponin.
abstract: Calponin inhibits actomyosin Mg2+ ATPase and is proposed to regulate smooth muscle contraction; however, the mechanism by which it exerts its effect and the regulation of its behavior is still under investigation. The proposed methods by which calponin regulation is effected include reversible phosphorylation of calponin which would allow contraction to occur and regulation by interaction with calcium-calmodulin. However, several investigators have been unable to find evidence of in vivo phosphorylation of calponin, and the affinity between calponin and calmodulin is not high enough to suggest that this interaction is biologically significant. In this paper, we present an alternative method of calponin regulation via calcium-caltropin and describe the calponin-caltropin complex for the first time. Caltropin, a calcium-binding protein isolated from smooth muscle, is a dimer under native conditions and interacts with calponin in a calcium-dependent fashion in the ratio of 2 mol of dimer: 1 mol of calponin. The formation of this complex can be monitored by following the fluorescence of an acrylodan label on cysteine 273 of calponin, which undergoes a 35-nm blue shift in wavelength peak from 505 to 470 nm when calponin becomes complexed with caltropin. This fluorescence change when titrated with calcium indicates that the concentration of calcium required for complex formation is approximately 10(-5) M, corresponding to the low-affinity calcium-binding sites of caltropin. This complex was further characterized by circular dichroism (CD).(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Biochemistry, University of Alberta, Edmonton, Canada.
authors.forename: F L
authors.initials: FL
authors.lastname: Wills
authors.forename: W D
authors.initials: WD
authors.lastname: McCubbin
authors.forename: C M
authors.initials: CM
authors.lastname: Kay
mesh_terms: D015081:2-Naphthylamine / Q000031:analogs & derivatives; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000737:chemistry; D002135:Calcium-Binding Proteins / Q000302:isolation & purification / Q000378:metabolism*; D018797:Cell Cycle Proteins; D002942:Circular Dichroism; D005456:Fluorescent Dyes; D019791:Guanidine; D006146:Guanidines; D006358:Hot Temperature; D008840:Microfilament Proteins; D009124:Muscle Proteins / Q000302:isolation & purification / Q000378:metabolism*; D009130:Muscle, Smooth / Q000378:metabolism*; D011489:Protein Denaturation; D000075383:S100 Calcium Binding Protein A6; D009418:S100 Proteins; D013050:Spectrometry, Fluorescence; D014422:Turkeys; D014461:Ultracentrifugation; D000096985:Calponins
chemical_list: D015081:2-Naphthylamine; D000251:Adenosine Triphosphatases; D002118:Calcium; D002135:Calcium-Binding Proteins; D018797:Cell Cycle Proteins; D005456:Fluorescent Dyes; D019791:Guanidine; D006146:Guanidines; D008840:Microfilament Proteins; D009124:Muscle Proteins; D000075383:S100 Calcium Binding Protein A6; D009418:S100 Proteins; D000096985:Calponins; C489133:S100A6 protein, Gallus gallus; C038518:acrylodan