Calcium-sensitive protein which reversibly blocks the barbed ends of actin filaments but does not sever preformed actin filaments. May play an important role in macrophage function. May play a role in regulating cytoplasmic and/or nuclear structures through potential interactions with actin. May bind DNA. Uncapping occurs either when Ca(2+) falls or when the concentration of polyphosphoinositide rises, both at low and high Ca(2+). Interacts with NUP62. Interacts with NUTF2 and RAN; involved in CAPG nuclear import. In macrophages, may be predominantly cytoplasmic. Nuclear localization was observed in fibroblasts. In macrophages, present at the membrane-cytoplasm interface. In activated macrophages, concentrated in the ruffles of the leading lamellipodia. Present in a large variety of tissues and is particularly abundant in kidney and lung. Highly expressed in macrophages (at protein level) (PubMed:8293478). Up-regulated during HL60 cell differentiation into macrophages (at protein level). Phosphorylated. Nuclear GCAP39 is more highly phosphorylated than cytoplasmic GCAP39. Belongs to the villin/gelsolin family. This protein was originally thought to be a DNA-binding protein with a helix-loop-helix domain.