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An extracellular matrix-associated glycoprotein expressed in a variety of
tissues during embryogenesis and repair, SPARC contains modular domains that can
function independently to bind cells and matrix components. Because SPARC can
selectively disrupt cellular contacts with matrix and thereby effect changes in
cell shape, it has been referred to as an antiadhesin. Inhibition of the
expression of SPARC altered axial development in frogs, and deregulated
expression in nematode worms resulted in a derangement of muscle attachment and
embryonic lethality. SPARC also inhibits cell cycle progression in vitro, in
part through a cationic, disulfide-bonded region that is homologous to a
repeated domain in the cytokine inhibitor, follistatin. Moreover, SPARC binds
specifically to the B chain of platelet-derived growth factor and alters the
response of cells to several cytokines. Although information concerning the
expression, biochemical properties, and cellular activities of SPARC has
increased significantly over the last decade, the precise function of the
protein has not been resolved. Goals of future studies include characterization
of cell-surface receptors for SPARC and the interactions with morphogens and
growth factors that regulate specific activities during animal development.
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