The beta-subunit of the protein-conducting channel of the endoplasmic reticulum functions as the guanine nucleotide exchange factor for the beta-subunit of the signal recognition particle receptor.
Cotranslational protein transport to the endoplasmic reticulum is controlled by the concerted interaction of three GTPases: the SRP54 subunit of the signal recognition particle (SRP) and the alpha- and beta-subunits of the SRP receptor (SR). SRbeta is related to ADP-ribosylation factor (ARF)-type GTPases, and the recently published crystal structure of SRbeta-GTP in complex with the binding domain of SRalpha suggested that SRbeta, like all ARF-type GT-Pases, requires a guanine nucleotide exchange factor (GEF) for function. Searching the sequence data base, we identified significant sequence similarity between the Sec7 domain of ARF-GEFs and the cytosolic domains of the beta-subunits of the two homologous heterotrimeric protein-conducting channels in yeast. Using a fluorescence nucleotide exchange assay, we show that the beta-subunits of the heterotrimeric protein-conducting channels function as the GEFs for SRbeta. Both the cytosolic domain of Sec61beta as well as the holo-Sec61beta, when part of the isolated trimeric Sec61p complex, function as the GEF for SRbeta, whereas the same Sec61beta, when part of the heptameric complex that facilitates posttranslational protein transport, is inactive as the GEF for SRbeta
Helmers
Jurgen
J
Laboratory of Cell Biology, Howard Hughes Medical Institute, The Rockefeller University, New York, New York 10021, USA. helmerj@blobelsgi.rockefeller.edu
Schmidt
Daniel
D
Glavy
Joseph S
JS
Blobel
Gunter
G
Schwartz
Thomas
T
D000595:Amino Acid Sequence; D004721:Endoplasmic Reticulum / Q000737:chemistry*; D020558:GTP Phosphohydrolases; D020662:Guanine Nucleotide Exchange Factors / Q000737:chemistry* / Q000502:physiology; D046911:Macromolecular Substances; D008565:Membrane Proteins / Q000737:chemistry* / Q000502:physiology; D026901:Membrane Transport Proteins / Q000737:chemistry / Q000502:physiology; D017434:Protein Structure, Tertiary; D021122:Protein Subunits; D021381:Protein Transport; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; D000069816:SEC Translocation Channels; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry / Q000502:physiology; D016415:Sequence Alignment; D033921:Vesicular Transport Proteins
D020662:Guanine Nucleotide Exchange Factors; D046911:Macromolecular Substances; D008565:Membrane Proteins; D026901:Membrane Transport Proteins; D021122:Protein Subunits; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; C101189:SBH2 protein, S cerevisiae; D000069816:SEC Translocation Channels; D029701:Saccharomyces cerevisiae Proteins; C101188:Sbh1 protein, S cerevisiae; D033921:Vesicular Transport Proteins; C036318:signal peptide receptor; D020558:GTP Phosphohydrolases