Transmembrane protein CD69 acts as an S1PR1 agonist

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• Sphingosine-1-phosphate receptor 1 (S1PR1) activation is crucial for lymphocyte egress, immune surveillance, and T cell activity.
• CD69 is an endogenous regulator that inhibits lymphocyte egress by interacting with S1PR1, but the mechanism was unclear.
• It was unknown if transmembrane proteins could act as agonists for class A GPCRs.

Purpose of the Study:

• To elucidate the structural mechanism by which CD69 regulates S1PR1.
• To determine if CD69 functions as a protein agonist for S1PR1.
• To understand the role of CD69-S1PR1 interaction in G-protein signaling and receptor internalization.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of CD69-bound S1PR1 complexed with a G_i protein.
• Site-directed mutagenesis to investigate key residues at the CD69-S1PR1 interface.
• Functional assays to assess receptor internalization and signaling.

Main Results:

• The cryo-EM structure revealed that the transmembrane helix of CD69 directly contacts S1PR1, inducing allosteric changes for G_i activation.
• Mutations at the interface disrupted CD69-S1PR1 interactions and reduced receptor internalization.
• CD69 was demonstrated to act as a *cis*-acting protein agonist of S1PR1.

Conclusions:

• CD69 functions as a *cis*-acting protein agonist for S1PR1, directly activating it.
• This interaction promotes G_i-dependent S1PR1 internalization, impairing S1P gradient sensing.
• The findings reveal a novel mechanism for inhibiting lymphocyte egress and have implications for autoimmune disease therapies.