Signaling processes in the cell tend to follow a familiar cycle—the activation and deactivation of membrane-bound receptors. Yet some signaling processes defy this cycle. They become activated and stay activated, preventing downstream cellular functions from returning to the status quo.
Such message control may well have its uses in sustaining health or yielding to illness, which is why it attracted the interest of scientists based at Duke University. Specifically, these scientists decided to study G-protein-coupled receptors (GPCRs) that seemed to counter foundational assumptions about cell signaling.
Classically, it was known that GPCRs located along the plasma membrane inside the cell activate G proteins, which are the molecular switches that transmit signals from external sources into the cell's interior, telling the cell how to function.
The activation process is followed by desensitization, led by a protein called β-arrestin that binds to the receptor, blocking further activation of G proteins and pulling the receptor to the inside of the cell in a process termed internalization or endocytosis. The end result of these two processes is to silence receptor signaling, allowing cellular function to return to baseline.
In recent years, however, scientists learned that some GPCRs continue to signal to G proteins, even after β-arrestin has been deployed and the receptors were internalized in the cellular compartments, called endosomes. These observations challenged the existing understanding of how GPCR signaling works.
In hopes of resolving this quirk of cell signaling, the Duke scientists employed a variety of biochemical, biophysical, and cell-based methods in a new study of GPCRs. Ultimately, these scientists, led by Robert Lefkowitz, M.D., a professor at Duke and a Howard Hughes Medical Institute investigator, uncovered the existence, functionality, and architecture of previously unknown superstructures of receptors, which they've called supercomplexes or “megaplexes.”
Details of this work appeared August 4 in the journal Cell, in an article entitled, “GPCR-G Protein-β-Arrestin Super-Complex Mediates Sustained G Protein Signaling.”
“These super-complexes or 'megaplexes' more readily form at receptors that interact strongly with β-arrestins via a C-terminal tail containing clusters of serine/threonine phosphorylation sites,” wrote the article’s authors. “Single-particle electron microscopy analysis of negative-stained purified megaplexes reveals that a single receptor simultaneously binds through its core region with G protein and through its phosphorylated C-terminal tail with β-arrestin.”
The megaplexes differ from the typical couplings of the receptors and β-arrestin, binding simultaneously through their core region with G protein and through a tail region with β-arrestin. Because β-arrestin interacts only with the receptor tail, the entire inner surface of the receptor is exposed, enabling the receptor to keep activating the G protein.
“The formation of such megaplexes explains how G proteins can continue to send signals after being internalized by GPCRs,” said Dr. Lefkowitz. “This opens a whole world of possibilities yet to be explored to manipulate this duality of signaling from outside and inside the cell for therapeutic benefit.”
Co-lead author Alex R.B. Thomsen added that some previous studies showed that the cells respond differently when G protein signaling occurs from different cellular compartments.
“As a result, pharmaceutical drugs developed in the future, if they are capable of regulating signaling at specific compartments, might be able to better treat certain diseases while having fewer side effects,” Dr. Thomsen noted. Such research is in its infancy, however, and clinical applications are years away.