Deciphering the multi-site phos-code of IRBIT underlying its binding to IP₃R.

Area of Science:

• Molecular Biology
• Cell Signaling
• Biochemistry

Background:

• IRBIT interacts with the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) through a serine-rich region.
• This interaction is phosphorylation-dependent, but specific phosphoisotypes involved remain unclear.

Purpose of the Study:

• To identify the specific phosphoisotypes of IRBIT involved in IP3R binding.
• To elucidate the mechanism by which IRBIT phosphorylation regulates IP3R activity.

Main Methods:

• In vitro kinase assays to identify phosphorylation sites.
• Pulldown assays to determine IP3R binding sites.
• Computational modeling of peptide-receptor interactions.
• Calcium (Ca2+) imaging in living cells to assess IP3R-mediated Ca2+ release.

Main Results:

• Serine 68 (S68) is the predominant phosphorylation site on IRBIT but is not essential for IP3R binding.
• Protein kinase A and casein kinase 2 phosphorylate distinct serine/threonine residues on IRBIT.
• Phosphorylated residues S71/S74/S77 and S80/S84/S85 form binding sites for the IP3-binding core (IBC) of IP3R.
• A peptide mimicking phosphorylated S80/S84/S85 on IRBIT binds the IBC similarly to IP3.
• Mutating S80, S84, and S85 to aspartate (mimicking phosphorylation) inhibited IP3R-mediated Ca2+ release.

Conclusions:

• Phosphorylation of IRBIT at S80, S84, and S85 creates binding sites for the IP3R.
• These phosphorylated residues likely compete with IP3 for the IP3-binding pocket on the IP3R.
• This competitive binding mechanism regulates IP3R channel activity and intracellular calcium release.