title: Purification and characterization of a guanine nucleotide-exchange protein for ADP-ribosylation factor from spleen cytosol.
abstract: ADP-ribosylation factors (ARFs) are 20-kDa guanine nucleotide-binding proteins and are active in the GTP-bound state and inactive with GDP bound. ARF-GTP has a critical role in vesicular transport in several cellular compartments. Conversion of ARF-GDP to ARF-GTP is promoted by a guanine nucleotide-exchange protein (GEP). We earlier reported the isolation from bovine brain cytosol of a 700-kDa protein complex containing GEP activity that was inhibited by brefeldin A (BFA). Partial purification yielded an approximately 60-kDa BFA-insensitive GEP that enhanced binding of ARF1 and ARF3 to Golgi membranes. GEP has now been purified extensively from rat spleen cytosol in a BFA-insensitive, approximately 55-kDa form. It activated class I ARFs (ARFs 1 and 3) that were N-terminally myristoylated, but not nonmyristoylated ARFs from class-I, II, or III. GEP activity required MgCl2. In the presence of 0.6-0.8 mM MgCl2 and 1 mM EDTA, binding of guanosine 5'-[gamma[35S]thio]triphosphate ([35S]GTP gamma S) by ARF1 and ARF3 was equally high without and with GEP. At higher Mg2+ concentrations, binding without GEP was much lower; with 2-5 mM MgCl2, GEP-stimulated binding was maximal. The rate of GDP binding was much less than that of GTP gamma S with and without GEP. Phospholipids were necessary for GEP activity; phosphatidylinositol was more effective than phosphatidylserine, and phosphatidic acid was less so. Other phospholipids tested were ineffective. Maximal effects required approximately 200 microM phospholipid, with half-maximal activation at 15-20 microM. Release of bound [35S]GTP gamma S from ARF3 required the presence of both GEP and unlabeled GTP or GTP gamma S; GDP was much less effective. This characterization of the striking effects of Mg2+ concentration and specific phospholipids on the purified BFA-insensitive ARF GEP should facilitate experiments to define its function in vesicular transport.
authors.affiliation: Pulmonary/Critical Care Medicine Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: S C
authors.initials: SC
authors.lastname: Tsai
authors.forename: R
authors.initials: R
authors.lastname: Adamik
authors.forename: J
authors.initials: J
authors.lastname: Moss
authors.forename: M
authors.initials: M
authors.lastname: Vaughan
mesh_terms: D020823:ADP-Ribosylation Factor 1; D020727:ADP-Ribosylation Factors; D000818:Animals; D003600:Cytosol / Q000737:chemistry; D019204:GTP-Binding Proteins / Q000378:metabolism*; D020662:Guanine Nucleotide Exchange Factors; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D015636:Magnesium Chloride / Q000494:pharmacology; D019814:Myristic Acid; D009227:Myristic Acids / Q000737:chemistry; D010743:Phospholipids / Q000737:chemistry; D011506:Proteins / Q000302:isolation & purification*; D051381:Rats; D013154:Spleen / Q000737:chemistry; D013329:Structure-Activity Relationship
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D009227:Myristic Acids; D010743:Phospholipids; D011506:Proteins; D015636:Magnesium Chloride; D019814:Myristic Acid; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D019204:GTP-Binding Proteins; D020823:ADP-Ribosylation Factor 1; D020727:ADP-Ribosylation Factors