Cysteine-rich protein I (CRPI) regulates actin filament bundling

Background: Cysteine-rich protein I (CRPI) is a LIM domain containing protein localized to the nucleus and the actin cytoskeleton. CRPI has been demonstrated to bind the actin-bundling protein alpha-actinin and proposed to modulate the actin cytoskeleton; however, specific regulatory mechanisms have not been identified. Results: CRPI expression increased actin bundling in rat embryonic fibroblasts. Although CRPI did not affect the bundling activity of alpha-actinin, CRPI was found to stabilize the interaction of alpha-actinin with actin bundles and to directly bundle actin microfilaments. Using confocal and photobleaching fluorescence resonance energy transfer ( FRET) microscopy, we demonstrate that there are two populations of CRPI localized along actin stress fibers, one associated through interaction with alpha-actinin and one that appears to bind the actin filaments directly. Consistent with a role in regulating actin filament cross-linking, CRPI also localized to the membrane ruffles of spreading and PDGF treated fibroblasts. Conclusion: CRPI regulates actin filament bundling by directly cross-linking actin filaments and stabilizing the interaction of alpha-actinin with actin filament bundles.