Calponin

Calponin is a family of actin-binding proteins that associates with thin filaments in smooth muscle and, to a lesser extent, in other cell types. It is encoded by three genes, CNN1, CNN2, and CNN3, producing isoforms commonly referred to as calponin-1, calponin-2, and calponin-3. Calponin proteins localize to the cytoskeleton, where they interact with actin and modulate the contractile apparatus in a tissue-dependent manner. Calponin-1 is most strongly expressed in differentiated smooth muscle, while calponin-2 is found in smooth muscle and various non-muscle cells, and calponin-3 is enriched in the brain and other tissues.

Functionally, calponin inhibits actin-activated myosin ATPase, thereby dampening the ability of myosin to generate force on actin filaments. This inhibitory effect contributes to the regulation of smooth muscle contraction and helps stabilize the cytoskeletal network. The activity of calponin is influenced by phosphorylation and interactions with signaling molecules such as calmodulin and kinases, which can modulate its binding to actin and its inhibitory impact on the contractile machinery.

In research and clinical contexts, calponin serves as a marker of smooth muscle differentiation and is used in studies of vascular biology, development, and pathology. Changes in calponin expression or distribution have been observed in vascular remodeling, cancer, and neural development, reflecting its role in cytoskeletal organization and contractile regulation. The different isoforms of calponin allow nuanced regulation across tissues, contributing to both muscle-specific and broader cellular functions.