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Smooth muscle contraction is regulated primarily by the reversible phosphorylation of myosin II (Somlyo & Somlyo, 1994), but may also be modulated by a thin filament-associated mechanism (Walsh, 1990). Calponin is a candidate for such a modulator (Winder et al. 1998). Calponin was first isolated from chicken gizzard smooth muscle as a 34 kDa actin- and calmodulin-binding protein (Takahashi et al. 1986) and later shown to exist as multiple isoforms. h1- (
or basic), h2- (neutral) and acidic calponins are the products of three distinct genes; 
-calponin is an alternatively spliced variant of h1-calponin. In adult vertebrates, h1-calponin expression is restricted to differentiated smooth muscle cells where it is localized to the contractile and cytoskeletal actin filaments. Although its physiological functions are not clear, h1-calponin has been implicated in the regulation of both contractile and non-contractile functions (adhesion, migration and proliferation). h1-Calponin expression is down-regulated when vascular smooth muscle cells re-enter the cell cycle and proliferate, changing from a contractile to a synthetic phenotype (as occurs in response to vascular injury), and h1-calponin is a useful marker of the contractile phenotype of smooth muscle cells. During embryonic development, this isoform is expressed in other tissues, including the heart, but disappears during late fetal development. h1-Calponin has been implicated in cardiac myofibrillar development and regulation of embryonic heart contraction. Its expression has also been reported in some transformed cells, e.g. osteosarcomas, and a role for the protein in tumour suppression has been proposed. Neutral (h2-) and acidic isoforms of calponin are expressed in smooth muscle and non-muscle cells, but at much lower levels, and less is known about their potential physiological roles. The acidic isoform has been implicated in neurite outgrowth and navigation in the rat and in early embryonic development in Xenopus. -Calponin has been detected at the protein level only in smooth muscle cells of the urogenital tract.
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