Membrane-cytoskeleton-associated protein with F-actin-binding activity that induces F-actin bundles formation and stabilization. Its F-actin-bundling activity is reversibly regulated upon its phosphorylation by the cAMP-dependent protein kinase A (PKA). Binds to the erythrocyte membrane glucose transporter-1 SLC2A1/GLUT1, and hence stabilizes and attaches the spectrin-actin network to the erythrocytic plasma membrane. Plays a role in maintaining the functional integrity of PKA-activated erythrocyte shape and the membrane mechanical properties. Also plays a role as a modulator of actin dynamics in fibroblasts; acts as negative regulator of the RhoA activation pathway. In platelets, functions as a regulator of internal calcium mobilization across the dense tubular system that affects platelet granule secretion pathways and aggregation. Also required for the formation of a diverse set of cell protrusions, such as filopodia and lamellipodia, necessary for platelet cell spreading, motility and migration. Acts as a tumor suppressor and inhibits malignant cell transformation. Monomeric (isoform 2); under reducing conditions. Self-associates. Exists under oxidizing condition as a trimer of two isoforms 2 and isoform 1 linked by disulfide bonds (Probable). Found in a complex with DMTN, F-actin and spectrin. Found in a complex with ADD2, DMTN and SLC2A1. Interacts with F-actin, ITPKB and spectrin. Isoform 2 interacts with SLC2A1 (via C-terminus cytoplasmic region) (By similarity). Interacts with RASGRF2. Localized at the spectrin-actin junction of erythrocyte plasma membrane. Localized to intracellular membranes and the cytoskeletal network. Localized at intracellular membrane-bounded organelle compartment in platelets that likely represent the dense tubular network membrane (By similarity). Expressed in platelets. Isoform 1 and isoform 2 are expressed in mature erythrocytes (at protein level). Both the N-terminal core domain and the C-terminal headpiece domain are sufficient for binding to F-actin and necessary for actin bundling activity. Phosphorylated. Phosphorylation at Ser-403 by PKA causes the C-terminal headpiece domain to associate with the N-terminal core domain, and leads to the inhibition of its actin bundling activity (By similarity). Mice are viable and born at the expected Mendelian ratio. Adult mice show compensated anemia and display mild microcytosis and spherocytosis. The erythrocyte plasma membrane association with the spectrin-actin skeleton is fragile and mechanically unstable. Belongs to the villin/gelsolin family.