Regulator of STK3/MST2 and STK4/MST1 in the Hippo signaling pathway which plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. The core of this pathway is composed of a kinase cascade wherein STK3/MST2 and STK4/MST1, in complex with its regulatory protein SAV1, phosphorylates and activates LATS1/2 in complex with its regulatory protein MOB1, which in turn phosphorylates and inactivates YAP1 oncoprotein and WWTR1/TAZ. Phosphorylation of YAP1 by LATS1/2 inhibits its translocation into the nucleus to regulate cellular genes important for cell proliferation, cell death, and cell migration. SAV1 is required for STK3/MST2 and STK4/MST1 activation and promotes cell-cycle exit and terminal differentiation in developing epithelial tissues. Plays a role in centrosome disjunction by regulating the localization of NEK2 to centrosomes, and its ability to phosphorylate CROCC and CEP250. In conjunction with STK3/MST2, activates the transcriptional activity of ESR1 through the modulation of its phosphorylation. Homodimer. Stabilized through interaction with STK3/MST2 or STK4/MST1. Interacts (via SARAH domain) with isoform 1 of NEK2. Interacts with ESR1 only in the presence of STK3/MST2. Interacts with WTIP and AJUBA. Ubiquitously expressed in adult tissues with highest expression in the pancreas, aorta and interventricular septum and lowest expression in skeletal muscle. Expression was higher in fetal than in the adult heart. Expressed in various cell lines. Note: This description may include information from UniProtKB.