| Calponin 1 is a thin filament–associated actin‑binding protein of the calponin family that is selectively and highly expressed in contractile smooth muscle cells and in myofibroblasts, where it participates in the fine‑tuning of smooth muscle contractility and cytoskeletal organization. The protein contains an N‑terminal calponin‑homology–like region and internal calponin repeats that mediate binding to F‑actin, calmodulin, and tropomyosin, and a C‑terminal region that modulates these interactions and provides docking sites for signaling proteins such as PKC and ERK. Calponin 1 associates with actin filaments both in the contractile apparatus, together with myosin and caldesmon, and in the cytoskeletal compartment, together with β‑cytoplasmic actin, filamin, and desmin, and is enriched in cytoplasmic dense bodies and adhesion plaques that anchor actin bundles to the membrane. Binding of calponin 1 to actin inhibits actin‑activated myosin Mg‑ATPase activity and slows actin filament sliding over myosin, providing an actin‑linked regulatory mechanism that complements myosin regulatory light chain phosphorylation in controlling the amplitude and duration of smooth muscle contraction. Calponin 1 also stabilizes actin filaments and decreases their flexural dynamics, contributing to maintenance of filament integrity and to the mechanical stiffness of actin networks that transmit and resist contractile and external forces. The protein interacts with signaling molecules including PKC isoforms, Raf, and ERK1/2 and can function as a scaffold that modulates agonist‑induced ERK activation and calcium sensitization in vascular smooth muscle, linking receptor‑proximal kinase cascades to alterations in contractile responsiveness. Expression of calponin 1 is characteristic of differentiated vascular and visceral smooth muscle and is induced during myofibroblast differentiation, making it a reliable marker for contractile smooth muscle and activated fibroblast lineages in development, wound repair, and fibrosis. Calponin 1 in negative regulation of vascular smooth muscle cell proliferation and maintains a contractile, non‑synthetic phenotype, connecting its cytoskeletal functions with control of vascular remodeling. Altered expression or localization of calponin 1 associates with vascular pathologies and with changes in tumor vasculature, where loss or redistribution of calponin‑positive contractile elements accompanies abnormal vessel structure and function. |
