Bundle formation of smooth muscle desmin intermediate filaments by calponin and its binding site on the desmin molecule

Smooth muscle basic calponin, a major actin-, tropomyosin-, and calmodulin- binding protein, has been examined for its ability to interact with desmin intermediate filaments from smooth muscle cells using sedimentation analysi s, turbidity changes, chemical cross-linking, matrix-assisted laser desorpt ion ionization time-of-flight mass spectrometry (MALDI TOF/MS), and electro n microscopic observations. Calponin interacted with desmin intermediate fi laments in a concentration-dependent manner in vitro. The binding of calpon in to desmin produced dense aggregates at 30 degrees C. The dense aggregate s were observed by electron microscopy to be composed of large anisotropic bundles of desmin filaments, indicating that calponin forms bundles of desm in filaments. The addition of calmodulin or S100 to the mixture of calponin and desmin caused the removal of calponin from the desmin filaments and in hibited bundle formation in the presence of Ca2+, but not in the presence o f EGTA. Calponin-related proteins including G-actin, tropomyosin, and SM22, had little effect on the binding of calponin to desmin filaments, whereas tubulin weakly inhibited the binding. Desmin had little influence on the ca lponin-actin and calponin-tubulin interactions using the zero-length cross- linker, EDC. Domain mapping with chymotryptic digestion showed that the bin ding site of calponin resides within the central cu-helical rod domain of t he desmin molecule. The chemical cross-linked products of calponin and synt hetic peptides (TQ27, TNEKVELQELNDRFANYIEKVRFLEQQ; EE24, EEELRELRRQVDALTGQR ARVEVE) derived from the rod domain were detected by MALDI TOF/MS. Furtherm ore, the calponin-desmin interaction was significantly inhibited by the add ition of EE24, but only slightly by TQ27. These results suggest that calpon in may act as a cross-linking protein between desmin filaments as well as a mong intermediate filaments, microfilaments and microtubules in smooth musc le cells.