Regulator of protein phosphatase 1 (PP1) that acts as a positive regulator of pulmonary endothelial cell (EC) barrier function. Protects the endothelial barrier from lipopolysaccharide (LPS)-induced vascular leakage (By similarity). Involved in the regulation of the PI3K/AKT signaling pathway (By similarity). Involved in the regulation of angiogenesis and endothelial cell proliferation through the control of ECE1 dephosphorylation, trafficking and activity (PubMed:26806547). Involved in the regulation of endothelial cell filopodia extension (PubMed:17609201). May be a downstream target for TGF-beta1 signaling cascade in endothelial cells (By similarity). Involved in PKA-mediated moesin dephosphorylation which is important in EC barrier protection against thrombin stimulation. Promotes the interaction of PPP1CA with RPSA/LAMR1 and in turn facilitates the dephosphorylation of RPSA/LAMR1 (By similarity). Involved in the dephosphorylation of EEF1A1 (By similarity). Interacts with PPP1CA, PPP1CB and MSN. Interacts (via its fourth ankyrin repeat) with the mature dimeric form of RPSA/LAMR1. Interacts with EEF1A1 (By similarity). Interacts with PTEN (By similarity). Interacts with ECE1 (PubMed:26806547). Colocalizes with RPSA/LAMR1 in the cell membrane (By similarity). Localizes to the perinuclear region (PubMed:12055102). Colocalizes with PTEN at the tip of EC projections (By similarity). Inhibited by TGF-beta1 (PubMed:12055102). Down-regulated by LPS (PubMed:21907835). Phosphorylated by PKA and, after PKA priming, by GSK3B. Phosphorylation by GSK3B reduces its association with PP1C and enhances PP1C activity. Dephosphorylation by its associated PP1C results in enhanced association with PP1C, but reduced PP1C activity.