Calponin, an actin- and Ca(2+)-calmodulin-binding protein characterized as an inhibitory factor of the smooth-muscle actomyosin activity, has also been shown to be present in some non-muscle cells. However, there is a controversy as to whether calponin is present or not in brain. Several laboratories indicate that this protein is absent in chicken or bovine brains, while Applegate et al. [Applegate, Feng, Green and Taubman (1994) J. Biol. Chem. 269, 10683-10690] have recently reported the identification of an mRNA specific for a 36 kDa non-muscle calponin analogue in homogenates of rat brains. For the first time we demonstrate, by Western blots and in situ immunofluorescence localization using monoclonal as well as affinity-purified polyclonal antibody to gizzard calponin, that a 36-37 kDa and a 35-36 kDa calponin-like proteins are expressed respectively in pig and rat brains and in rat cerebellar cultured cells. The acidic pI (5.2-5.4) of the rat brain protein revealed by isoelectric focusing is in good agreement with that of the protein coded for by the calponin isoform mRNA described by Applegate et al. and is different from that of the protein from chicken gizzard (pI 9.9). Brain calponin-like protein is different from two other Ca(2+)-calmodulin-binding proteins previously identified in brain, namely caldesmon and adducin, and from tropomyosin.

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