Engulfment and subsequent degradation of apoptotic cells is an essential step that occurs throughout life in all multicellular organisms [1][2][3] . ELMO/Dock180/Rac proteins are a conserved signalling module for promoting the internalization of apoptotic cell corpses 4,5 ; ELMO and Dock180 function together as a guanine nucleotide exchange factor (GEF) for the small GTPase Rac, and thereby regulate the phagocyte actin cytoskeleton during engulfment [4][5][6] . However, the receptor(s) upstream of the ELMO/ Dock180/Rac module are still unknown. Here we identify brainspecific angiogenesis inhibitor 1 (BAI1) as a receptor upstream of ELMO and as a receptor that can bind phosphatidylserine on apoptotic cells. BAI1 is a seven-transmembrane protein belonging to the adhesion-type G-protein-coupled receptor family, with an extended extracellular region 7-9 and no known ligands. We show that BAI1 functions as an engulfment receptor in both the recognition and subsequent internalization of apoptotic cells. Through multiple lines of investigation, we identify phosphatidylserine, a key 'eat-me' signal exposed on apoptotic cells 10-13 , as a ligand for BAI1. The thrombospondin type 1 repeats within the extracellular region of BAI1 mediate direct binding to phosphatidylserine. As with intracellular signalling, BAI1 forms a trimeric complex with ELMO and Dock180, and functional studies suggest that BAI1 cooperates with ELMO/Dock180/Rac to promote maximal engulfment of apoptotic cells. Last, decreased BAI1 expression or interference with BAI1 function inhibits the engulfment of apoptotic targets ex vivo and in vivo. Thus, BAI1 is a phosphatidylserine recognition receptor that can directly recruit a Rac-GEF complex to mediate the uptake of apoptotic cells.Previous studies revealed two 'functional' regions within ELMO1 and its Caenorhabditis elegans homologue CED-12 during phagocytosis 5,14-17 . The amino-terminal 558 amino-acid residues (N-term) were necessary for targeting of the ELMO-Dock180 complex to the membrane 14,17 , whereas the carboxy-terminal 196 residues (C-term) were necessary for binding Dock180 and for optimal Rac activation 15,16 . Because the receptor(s) upstream of ELMO1 during engulfment were not known, we performed a yeast two-hybrid screen, with N-term as bait. After screening more than 1.1 3 10 7 colonies from a mouse embryo library, followed by several subscreens for specificity, we identified a single membrane protein, BAI1.BAI1 belongs to subgroup VII of the adhesion-type G-proteincoupled receptor (GPCR) family 7-9 , with extended extracellular termini containing multiple domains and motifs that are thought to function in cell-cell or cell-matrix interactions 9 . BAI1 (1,584 residues) has an 943-residue extracellular region, a seven-transmembrane

Mammalian Dock180 and ELMO proteins, and their homologues in Caenorhabditis elegans and Drosophila melanogaster, function as critical upstream regulators of Rac during development and cell migration. The mechanism by which Dock180 or ELMO mediates Rac activation is not understood. Here, we identify a domain within Dock180 (denoted Docker) that specifically recognizes nucleotide-free Rac and can mediate GTP loading of Rac in vitro. The Docker domain is conserved among known Dock180 family members in metazoans and in a yeast protein. In cells, binding of Dock180 to Rac alone is insufficient for GTP loading, and a Dock180 ELMO1 interaction is required. We can also detect a trimeric ELMO1 Dock180 Rac1 complex and ELMO augments the interaction between Dock180 and Rac. We propose that the Dock180 ELMO complex functions as an unconventional two-part exchange factor for Rac.

Considerable data support the idea that Foxo1 drives the liver transcriptional program during fasting and is inhibited by Akt after feeding. Mice with hepatic deletion of Akt1 and Akt2 were glucose intolerant, insulin resistant, and defective in the transcriptional response to feeding in liver. These defects were normalized upon concomitant liver–specific deletion of Foxo1. Surprisingly, in the absence of both Akt and Foxo1, mice adapted appropriately to both the fasted and fed state, and insulin suppressed hepatic glucose production normally. Gene expression analysis revealed that deletion of Akt in liver led to constitutive activation of Foxo1–dependent gene expression, but once again concomitant ablation of Foxo1 restored postprandial regulation, preventing its inhibition of the metabolic response to nutrient intake. These results are inconsistent with the canonical model of hepatic metabolism in which Akt is an obligate intermediate for insulin’s actions. Rather they demonstrate that a major role of hepatic Akt is to restrain Foxo1 activity, and in the absence of Foxo1, Akt is largely dispensable for hepatic metabolic regulation in vivo.