Does calponin interact with caldesmon?
The roles of calponin and caldesmon and their interaction in regulation of smooth muscle contraction are controversial. Recently, strong binding between these two proteins has been reported (Graceffa, P., Adam, L. P., and Morgan, K. G. (1996) J. Biol. Chem. 271, 30336-30339). Results in this paper fail to confirm their data and are consistent with the concept of independent functions for calponin and caldesmon. To examine the ability of duck gizzard caldesmon to interact with calponin, three caldesmon derivatives, each containing a different sulfhydryl-specific reporter probe (6-acryloyl-2-dimethylaminonaphtalene, N-(1-pyrenyl)iodoacetamide, and N-iodoacetyl-N'-(5-sulfo-1-naphtylo)ethylenediamine) attached to a single cysteine located in the C-terminal domain, were synthesized. Addition of calponin to labeled caldesmon at both low and physiological salt concentrations did not induce any changes in fluorescence intensity or maximum shift. Under the same conditions, calmodulin and tropomyosin (known to bind to the C terminus of caldesmon) produced substantial changes in these spectral parameters. Gel filtration of an equimolar caldesmon-calponin mixture on a fast protein liquid chromatography Superose-12 column revealed two base-line-separated peaks, the first containing only caldesmon and the second only calponin, thus confirming the lack of any interaction between these two proteins. Also, the addition of calponin did not change the fluorescence parameters of labeled caldesmon in complexes with F-actin and F-actin-tropomyosin.