Vps74 gives phosphatase directions

Vps74 gives phosphatase directions P hosphoinositides have a crucial role in specifying the identity of different cellular compartments. In the Golgi apparatus, for example, phosphati-dylinositol 4-phosphate (PtdIns4P) is specifi cally generated at the trans-Golgi network (TGN) by phosphatidylinositol 4-kinase and is removed from other Golgi compartments by the lipid phosphatase Sac1. Sac1 is an integral membrane protein that mainly localizes to the endoplas-mic reticulum. Its phosphatase domain is tethered to the membrane by a long unstruc-tured linker, so how it fi nds its substrate on other organelles such as the Golgi is unclear. Cai et al. reveal that the PtdIns4P-binding protein Vps74 acts as a membrane receptor for Sac1, directing the phosphatase to early Golgi membranes to restrict the distribution of PtdIns4P and maintain the distinct identities of Golgi cisternae (1). PtdIns4P recruits the budding yeast protein Vps74 (a homologue of mamma-lian GOLPH3) to the TGN. The protein packages Golgi-resident glycosyltransferases into retrograde transport ves-icles that return the enzymes to the cis-and medial-Golgi cisternae (2, 3). " Yeast lacking Vps74 fail to retain these enzymes in the Golgi and instead deliver them to the vacuole where they're degraded , " explains Christo-pher Burd from Yale University School of Medicine in New Haven, Connecticut. Retrograde vesicles also transport PtdIns4P from the TGN to earlier Golgi compartments. In 2012, Burd and colleagues found that Vps74 binds to Sac1 and that deleting either of these two proteins elevated the level of PtdIns4P on medial-Golgi membranes (4). " We proposed that Vps74 grabs onto Sac1 and delivers it to the early Golgi so that it can scrub PtdIns4P from the membrane, " says Burd. Testing this hypothesis was diffi cult, however, because Vps74 interacts with many different Golgi proteins. Burd and colleagues, led by Yiying Cai and Yongqiang Deng, therefore sought to identify mutations that inhibited Vps74's association with Sac1 without affecting its interactions with other proteins or with PtdIns4P. Cai et al. determined the crystal structure of the complex formed between Vps74 and the N-terminal portion of Sac1, which contains both the phosphatase's catalytic domain and an additional, well-conserved, N-terminal subdomain (1). The researchers discovered that Vps74 interacts with the N-terminal subdomain of Sac1 and identifi ed point mutations in Vps74 that specifi cally disrupted this interaction. Yeast expressing this Vps74 mutant showed increased PtdIns4P levels at the medial Golgi, supporting the idea that Vps74 acts as a membrane receptor for Sac1. Moreover, …