An interaction between extracellular regulated kinase 1 (ERK1) and calponin has previously been reported (Menice, Hulvershorn, Adam, Wang and Morgan (1997) J. Biol. Chem.272(40), 25157-25161) and has been suggested to reflect a function of calponin as a signalling molecule. We report in this study that calponin binds to both ERK1 and ERK2 under native conditions as well as in an overlay assay. Using chymotryptic fragments of calponin, the binding site of ERK on calponin was identified as the calponin homology (CH) domain, an N-terminal region of calponin found in other actin-binding proteins. ERK also bound, in a gel overlay assay, α-actinin, a protein with two tandem CH domains, as well as a 27 kDa thermolysin product of α-actinin containing the CH domains of α-actinin. The CH domain of calponin could compete with intact calponin or α-actinin for ERK binding. Titration of acrylodan-labelled calponin with ERK gave a Ka of 6×106 M-1 and titration of acrylodan-labelled calponin with a peptide from the αL16 helix of ERK gave a Ka of 1×106 M-1. Recombinant ERK was found to co-sediment with purified actin and induced a fluorescence change in pyrene-labelled F-actin (Ka = 5×106 M-1). The interaction of ERK with CH domains points to a new potential function for CH domains. The interaction of ERK with actin raises the possibility that actin may provide a scaffold for ERK signalling complexes in both muscle and non-muscle cells.

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