Key regulatory enzyme is a molecular 'octopus'

of constant stimulation," said Lefkowitz. "Years ago, we had discovered that this down-regulation occurs due to a phosphorylation of the activated receptor that triggers binding of a protein called beta arrestin. This protein stops further G protein signaling and acts as an adaptor and scaffolding that connects to other signaling molecules."

Thus, the cellular "stop signal" not only turns off the G protein, but immediately tags the receptor for recycling into the cell interior and turns on other signaling pathways, said Lefkowitz. His laboratory identified that enzyme as GRK, but a central mystery was how the family of GRK enzymes fulfills their intricate regulatory duties.

In the latest work the researchers deduced the structure of GRK2, the member of the GRK enzyme family that is active in heart muscle and many other tissues.

Critical to solving that mystery was obtaining the three-dimensional structure of GRK2 using X-ray crystallography. In this technique, pure crystals of a protein are bombarded by an intense X-ray beam, and the protein structure is deduced by analyzing the pattern of the beam's diffraction. This structural determination was done by co-author John Tesmer and his colleagues.

The resulting structure revealed the details of three regions, or domains, of the GRK2 enzyme, which had earlier been identified by biochemical studies in the Lefkowitz laboratory:

  • The central, or catalytic domain is the region that triggers the phosphorylation reaction
  • The "regulator of G protein signaling homology" (RH) domain attaches to the G protein to switch it off, and
  • The "PH" domain enables GRK2 to home in on the G protein at the cell membrane and attach to it.

To reveal how GRK2 interacts with the G protein, the researchers obtained the structure of GRK2 attached to a subunit of the G protein to which it normally binds, or complexes. Lefkowitz noted that a particularly striking achievem

Contact: Dennis Meredith
Duke University

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