Mukherjee I, Barlowe C. Overexpression of Sly41 suppresses COPII vesicle-tethering deficiencies by elevating intracellular calcium levels.
Mol Biol Cell 2016;
27:1635-49. [PMID:
27030673 PMCID:
PMC4865320 DOI:
10.1091/mbc.e15-10-0704]
[Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2015] [Accepted: 03/22/2016] [Indexed: 11/29/2022] Open
Abstract
SLY41 is a multicopy suppressor of mutations in the essential Ypt1 GTPase. Overexpression of Sly41 elevates cytosolic Ca2+ concentration, which stimulates SNARE-dependent fusion of COPII vesicles with Golgi membranes and suppresses deficiencies in Ypt1-dependent vesicle tethering. Thus Ca2+ positively regulates vesicle fusion with Golgi membranes.
SLY41 was identified as a multicopy suppressor of loss of Ypt1, a Rab GTPase essential for COPII vesicle tethering at the Golgi complex. SLY41 encodes a polytopic membrane protein with homology to a class of solute transporter proteins, but how overexpression suppresses vesicle-tethering deficiencies is not known. Here we show that Sly41 is efficiently packaged into COPII vesicles and actively cycles between the ER and Golgi compartments. SLY41 displays synthetic negative genetic interactions with PMR1, which encodes the major Golgi-localized Ca2+/Mn2+ transporter and suggests that Sly41 influences cellular Ca2+ and Mn2+ homeostasis. Experiments using the calcium probe aequorin to measure intracellular Ca2+ concentrations in live cells reveal that Sly41 overexpression significantly increases cytosolic calcium levels. Although specific substrates of the Sly41 transporter were not identified, our findings indicate that localized overexpression of Sly41 to the early secretory pathway elevates cytosolic calcium levels to suppress vesicle-tethering mutants. In vitro SNARE cross-linking assays were used to directly monitor the influence of Ca2+ on tethering and fusion of COPII vesicles with Golgi membranes. Strikingly, calcium at suppressive concentrations stimulated SNARE-dependent membrane fusion when vesicle-tethering activity was reduced. These results show that calcium positively regulates the SNARE-dependent fusion stage of ER–Golgi transport.
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