ABSTRACT
Dynamin-related GTPase proteins (DRPs) are main players in membrane remodelling. Conserved DRPs called mitofusins (Mfn1/Mfn2/Fzo1) mediate the fusion of mitochondrial outer membranes (OM). OM fusion depends on self-assembly and GTPase activity of mitofusins as well as on two other proteins, Ugo1 and Mdm30. Here, we define distinct steps of the OM fusion cycle using in vitro and in vivo approaches. We demonstrate that yeast Fzo1 assembles into homo-dimers, depending on Ugo1 and on GTP binding to Fzo1. Fzo1 homo-dimers further associate upon formation of mitochondrial contacts, allowing membrane tethering. Subsequent GTP hydrolysis is required for Fzo1 ubiquitylation by the F-box protein Mdm30. Finally, Mdm30-dependent degradation of Fzo1 completes Fzo1 function in OM fusion. Our results thus unravel functions of Ugo1 and Mdm30 at distinct steps during OM fusion and suggest that protein clearance confers a non-cycling mechanism to mitofusins, which is distinct from other cellular membrane fusion events.
Subject(s)
F-Box Proteins/metabolism , GTP Phosphohydrolases/metabolism , Membrane Fusion , Membrane Proteins/metabolism , Mitochondrial Membranes/metabolism , Mitochondrial Proteins/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae/physiology , Amino Acid Sequence , Dimerization , F-Box Proteins/chemistry , F-Box Proteins/genetics , GTP Phosphohydrolases/chemistry , GTP Phosphohydrolases/genetics , Guanosine Triphosphate/metabolism , Membrane Proteins/chemistry , Membrane Proteins/genetics , Mitochondrial Membranes/chemistry , Mitochondrial Proteins/chemistry , Mitochondrial Proteins/genetics , Molecular Sequence Data , Protein Binding , Saccharomyces cerevisiae/chemistry , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae Proteins/chemistry , Saccharomyces cerevisiae Proteins/genetics , Sequence AlignmentABSTRACT
Over a hundred proteins in eukaryotic cells carry a C-terminal CaaX box sequence, which targets them for posttranslational isoprenylation of the cysteine residue. This modification, catalyzed by either farnesyl or geranylgeranyl transferase, converts them into peripheral membrane proteins. Isoprenylation is usually followed by proteolytic cleavage of the aaX tripeptide and methylation of the carboxyl group of the newly exposed isoprenylcysteine. The C-terminal modification regulates the cellular localization and biological activity of isoprenylated proteins. We have established a strategy to produce and purify recombinant farnesylated guanylate-binding protein 1 (hGBP1), a dynamin-related large GTPase. Our system is based on the coexpression of hGBP1 with the two subunits of human farnesyltransferase in Escherichia coli and a chromatographic separation of farnesylated and unmodified protein. Farnesylated hGBP1 displays altered GTPase activity and is able to interact with liposomes in the activated state.
