PKGI domain nuclear immunoreactivity in SMC. (A) PKGI has a functional leucine zipper (LZ) domain, cGMP-binding regulatory domain (REG), and catalytic region (CR), which were examined using the indicated antibodies. (B) In the nucleus of rat pulmonary artery smooth muscle cells (PASMC) and indicated SMC lines, PKGICR immunoreactivity was observed although PKGILZ reactivity was not detected. (C) Recombinant PKGI was also localized in SMC nuclei. RFL6 cells expressing PKGIβ-FLAG and GFP with a nuclear localization sequence (GFPNLS) were treated with digitonin, which permits soluble cytosolic PKGI to diffuse from the cell. PKGIβ-FLAG was detected in the nucleus with an anti-FLAG antibody (αFLAG) and co-localized with the GFPNLS epifluorescence.