Establishing and maintaining homeostasis is critical to the well-being of an organism and is determined by the balance of cell proliferation and death. Two genes that function together to regulate growth, proliferation, and apoptosis in Drosophila are warts (wts), encoding a serine/threonine kinase, and salvador (sav), encoding a WW domain containing Wts-interacting protein. However, the mechanisms by which sav and wts regulate growth and apoptosis are not well understood. Here, we describe mutations in hippo (hpo), which encodes a protein kinase most related to mammalian Mst1 and Mst2. Like wts and sav, hpo mutations result in increased tissue growth and impaired apoptosis characterized by elevated levels of the cell cycle regulator cyclin E and apoptosis inhibitor DIAP1. Hpo, Sav, and Wts interact physically and functionally, and regulate DIAP1 levels, likely by Hpo-mediated phosphorylation and subsequent degradation. Thus, Hpo links Sav and Wts to a key regulator of apoptosis.

The How of embryonic tissues during morphogenesis has been ascribed to tissue interfacial tensions arising from cell adhesion. Testing this explanation requires the demonstration that tissue interfacial tensions are measurable physical quantities. We describe a device that continuously records all parame-

The specificity of ubiquitin-mediated protein degradation with regards to the selection of substrates to be polyubiquitinated has only been determined rather recently. Substrate targeting by the N-end rule and HECT (homology to E6AP carboxyl terminus) domain ubiquitin ligases occurs through substrate-specific binding domains. In contrast, the SCF complex recruits substrates through a substrate adaptor protein, the F-box subunit. Despite evidence showing that Cdc20 and Cdh1 bind and activate the anaphase-promoting complex (APC) in a substrate-specific manner, there is no evidence that the activating protein and substrate interact directly; hence, no clear model exists for the mechanism of APC activation or recruitment of substrates. We show here that the activators Cdc20 and Cdh1 can associate with substrates via their N termini. In the absence of APC, Cdc20 and Cdh1 bind substrates reflecting Cdc20-APC and Cdh1-APC specificity. The N termini of Cdc20 and Cdh1 provide specificity functionally, as demonstrated by the generation of active chimeras that display the specificity corresponding to their N termini. Thus, Cdc20 and Cdh1 act as both substrate recognition and activating modules for APC. Ubiquitin-mediated protein degradation regulates important cellular processes from cell cycle progression to Wnt signaling (Peters 1998; for reviews, see Zachariae and Nasmyth 1999;Jackson et al. 2000). Attachment of a polyubiquitin chain to a protein targets that protein for degradation by the 26S proteasome (Coux et al. 1996;Baumeister et al. 1998). Ubiquitin becomes covalently attached to a substrate by interaction with a cascade of enzymes. The last in the series, a ubiquitin protein ligase (E3), transfers the ubiquitin to a lysine on the substrate protein (for review, see Hershko and Ciechanover 1998).In the N-end rule pathway, extending from bacteria to humans, a destabilizing residue at the N terminus directs proteins for ubiquitination via the E3 N-recognin/ Ubr1p (Saccharomyces cerevisiae) or E3␣ (in mammals). Direct binding of substrates to N-recognin/E3␣ is observed (Reiss et al. 1988;Gonda et al. 1989;Bartel et al. 1990; for review, see Varshavsky 1996). Similarly, HECT (homology to E6AP carboxyl terminus) domain E3s are highly conserved from yeast to vertebrates. They share homology in their C termini to E6AP and contain a conserved C-terminal cysteine residue that forms a thioester with ubiquitin; they bind substrates via their N termini (for review, see Jackson et al. 2000). E6AP targets the src kinases (Oda et al. 1999), HHR23A and HHR23B (Kumar et al. 1999). The HECT domain RSP5 in yeast targets Gap1 and Fur4 by interaction with its N-terminal WW domains (Hein et al. 1995;Rotin 1998;Wang et al. 1999). The fission yeast pub1 targets Cdc25 (Nefsky and Beach 1996). Xsmurf1 binds and targets Smad1 for degradation (Zhu et al. 1999).Two ubiquitin protein ligases play a role in the cell cycle: the SCF (Skp1/Cullin/F-box) complex and the anaphase-promoting complex (APC) (Peters 1998; for reviews, see Zachariae and ...

Activation of the Hippo pathway effector Yap underlies many liver cancers, however no germline or somatic mutations have been identified. Autophagy maintains essential metabolic functions of the liver, and autophagy-deficient murine models develop benign adenomas and hepatomegaly, which have been attributed to activation of the p62/Sqstm1-Nrf2 axis. Here, we show that Yap is an autophagy substrate and mediator of tissue remodeling and hepatocarcinogenesis independent of the p62/Sqstm1-Nrf2 axis. Hepatocyte-specific deletion of Atg7 promotes liver size, fibrosis, progenitor cell expansion, and hepatocarcinogenesis, which is rescued by concurrent deletion of Yap. Our results shed new light on mechanisms of Yap degradation and the sequence of events that follow disruption of autophagy, which is impaired in chronic liver disease.