An inositolphosphate-binding immunophilin, IPBP12

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Immunophilins are proteins that bind to immunosuppressants and possess peptidylprolyl cis-trans isomerase activity.
• FK506-binding proteins (FKBPs) are a class of immunophilins, with FKBP12 being a well-characterized cytosolic member.
• Inositol phosphates (IPs) are crucial signaling molecules involved in various cellular processes.

Purpose of the Study:

• To identify and characterize novel inositol phosphate-binding proteins.
• To investigate the enzymatic activity and ligand-binding properties of a newly discovered membrane-associated protein.
• To elucidate the potential cellular functions and interactions of this protein.

Main Methods:

• Isolation and purification of proteins from human erythrocyte and K562 cell membranes.
• Assay of peptidylprolyl cis-trans isomerase activity and inhibition studies using FK506, rapamycin, and various inositol phosphates.
• Protein sequencing of tryptic peptides and immunoprecipitation assays to identify interacting proteins.

Main Results:

• A novel 12-kD inositol phosphate-binding protein (IPBP12) was identified, exhibiting FKBP-like activity.
• IPBP12's isomerase activity was inhibited by FK506 and rapamycin, but uniquely by inositol 1,4,5-trisphosphate (IP(3)) and inositol 1,3,4,5-tetrakisphosphate (IP(4)).
• IPBP12 was found to associate with a protein kinase and phosphoprotein phosphatase 2A (PP-2A), and sequencing suggested cytoskeletal localization.

Conclusions:

• A novel membrane-associated immunophilin, IPBP12, binds both inositol phosphates and FK506-like compounds.
• IPBP12's interaction with IPs and its association with kinase/phosphatase complexes suggest a role in regulating cellular signaling pathways.
• Immunophilins may function in assembling multiprotein complexes involved in signal transduction and cellular regulation.