Homodimerization of the G protein SRbeta in the nucleotide-free state involves proline cis/trans isomerization in the switch II region.

Protein translocation across and insertion into membranes is essential to all life forms. Signal peptide-bearing nascent polypeptide chains emerging from the ribosome are first sampled by the signal-recognition particle (SRP), then targeted to the membrane via the SRP receptor (SR), and, finally, transferred to the protein-conducting channel. In eukaryotes, this ...
process is tightly controlled by the concerted action of three G proteins, the 54-kD subunit of SRP and the alpha- and beta-subunits of SR. We have determined the 2.2-A crystal structure of the nucleotide-free SRbeta domain. Unexpectedly, the structure is a homodimer with a highly intertwined interface made up of residues from the switch regions of the G domain. The remodeling of the switch regions does not resemble any of the known G protein switch mechanisms. Biochemical analysis confirms homodimerization in vitro, which is incompatible with SRalpha binding. The switch mechanism involves cis/trans isomerization of a strictly conserved proline, potentially implying a new layer of regulation of cotranslational transport.
Mesh Terms:
Amino Acid Sequence, Animals, Crystallography, X-Ray, Dimerization, Humans, Isomerism, Models, Molecular, Molecular Sequence Data, Proline, Protein Structure, Quaternary, Protein Subunits, Receptors, Cytoplasmic and Nuclear, Receptors, Peptide, Saccharomyces cerevisiae, Sequence Alignment
Proc. Natl. Acad. Sci. U.S.A.
Date: May. 02, 2006
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