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The Golgi Arf‐GEFs Gea1 and Gea2 Integrate Signals to Coordinate Vesicle Formation
Author(s) -
Gustafson Margaret Ann,
Fromme J. Christopher
Publication year - 2017
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.31.1_supplement.619.23
Subject(s) - golgi apparatus , copi , microbiology and biotechnology , gtpase , adp ribosylation factor , guanine nucleotide exchange factor , small gtpase , vesicle , chemistry , biology , secretory pathway , endoplasmic reticulum , biochemistry , signal transduction , membrane
A major challenge for eukaryotic cells is regulating the transport of protein cargoes among compartments and to and from the plasma membrane. How does a cell guarantee the efficient trafficking of appropriate cargoes to the only the right place at the right time? We and others have shown that at the Golgi complex, the cell's sorting house, decisions about packaging cargo rely on coordination of multiple signals. In Saccharomyces cerevisiae (and similarly in other eukaryotes including humans), this coordination relies heavily on small GTPases. The GTPase Arf1 acts as a central molecular switch: when switched on (GTP‐bound), it recruits cargoes, adapters, and coat proteins needed to form a vesicle at the Golgi. Three homologous proteins, called g uanine nucleotide ex change f actors (Arf‐GEFs), are responsible for exchanging GTP for GDP on Arf1 at the Golgi to activate the molecular switch. Sec7 (BIG1/2 in humans) turns on Arf1 to package secretory cargoes at the trans‐Golgi network (TGN), while Gea1 and Gea2 (GBF1 in humans) help Arf1 package retrograde cargoes into COPI vesicles for intra‐Golgi and Golgi‐to‐ER traffic. We have previously shown that Sec7 utilizes autoinhibition, positive feedback, and interactions with multiple small GTPases to regulate its activity and localization. Identifying the corresponding information used by Gea1/2 to make the decision about when and where to activate Arf1 is crucial to understanding how trafficking is regulated between the three Golgi Arf‐GEFs. We have identified three players in the decision‐making process for Gea1/2. First, all C‐terminal domains of each protein are required for regulating recruitment of the GEFs to membranes in vitro and some are required for viability of cells or cellular localization, highlighting the complex nature of coordinated signals. Second, the composition of the membrane surface is important: unlike Sec7, Gea1/2 prefer neutral to negatively‐charged membranes in vitro , paralleling the more neutral early Golgi compartments and negatively‐charged, phosphatidylserine‐rich TGN in vivo . Lastly, the Rab GTPase Ypt1 (Rab1 in humans) aids in recruiting Gea1/2 to membranes in vitro , in agreement with prior findings that overexpression of Ypt1 can suppress temperature sensitive mutations in Gea1/2. Together, these results begin to reveal the careful orchestration of retrograde Golgi membrane trafficking through coincidence detection. By sensing multiple signals for activation and recruitment, the Golgi Arf‐GEFs are able to carefully calibrate their activity and localization to achieve specificity in cargo packing and timing of vesiculation. Our ongoing work focuses on identifying the signals that fine‐tune compartment specificity within the Golgi and understanding the molecular mechanisms of these complementary interactions. Support or Funding Information This work was funded by the NIH (T32 GM07273 and R01 GM098621) and by the Harry and Samuel Mann Outstanding Graduate Student Award.