Insertion of circularly permuted cyan fluorescent protein into the ligand-binding domain of inositol 1,4,5-trisphosphate receptor for enhanced FRET upon binding of fluorescent ligand

• Published in: Cell calcium (Edinburgh) 2022-12, Vol.108, p.102668-102668, Article 102668
• Main Authors: Jahan, Azmeree, Akter, MST Tahmina, Takemoto, Kiwamu, Oura, Tai, Shitara, Akiko, Semba, Shingo, Nezu, Akihiro, Suto, Satoshi, Nagai, Takeharu, Tanimura, Akihiko
• Format: Article
• Language: English
• Subjects: Circularly permuted fluorescent protein; Fluorescence (förster) resonance energy transfer; Fluorescence Resonance Energy Transfer - methods; Fluorescent ligand; FRET; Inositol; Inositol 1,4,5-Trisphosphate - metabolism; Inositol 1,4,5-trisphosphate receptor; Inositol 1,4,5-Trisphosphate Receptors - metabolism; Ligand binding; Ligands; Protein Binding
• ISSN: 0143-4160; 1532-1991
• DOI: 10.1016/j.ceca.2022.102668

•Circularly permuted CFPs (cpCs) were inserted into the loop between the second and third α-helices of the ligand binding domain of IP3 receptor.•Two of the fluorescent ligand-binding proteins showed correct localization and exhibited FRET upon binding of the high-affinity ligand fluorescent adenophostin A (F-ADA) and the fluorescent low-affinity ligand (F-LL).•The effect of F-LL on the decrease in fluorescence ratio was reversed upon addition of IP3 by competing with IP3. Binding of fluorescent ligand (FL) to the cyan fluorescent protein (CFP)-coupled ligand-binding domain of the inositol 1,4,5-trisphosphate (IP3) receptor (CFP-LBP) produces fluorescence (Förster) resonance energy transfer (FRET). A competitive fluorescent ligand assay (CFLA), using the FRET signal from competition between FLs and IP3, can measure IP3 concentration. The FRET signal should be enhanced by attaching a FRET donor to an appropriate position. Herein, we inserted five different circularly permuted CFPs in the loop between the second and third α-helices to generate membrane-targeted fluorescent ligand-binding proteins (LBPs). Two such proteins, LBP-cpC157 and LBP-cpC173, localized at the plasma membrane, displayed FRET upon binding the high-affinity ligand fluorescent adenophostin A (F-ADA), and exhibited a decreased fluorescence emission ratio (480 nm / 535 nm) by 1.6- to 1.8-fold that of CFP-LBP. In addition, binding of a fluorescent low-affinity ligand (F-LL) also reduced the fluorescence ratio in a concentration-dependent manner, with EC50 values for LBP-cpC157 and LBP-cpC173 of 34.7 nM and 27.6 nM, respectively. These values are comparable to that with CFP-LBP (29.2 nM), indicating that insertion of cpC157 and cpC173 did not disrupt LBP structure and function. The effect of 100 nM F-LL on the decrease in fluorescence ratio was reversed upon addition of IP3, indicating binding competition between F-LL and IP3. We also constructed cytoplasmic fluorescent proteins cyLBP-cpC157 and cyLBP-cpC173, and bound them to DYK beads for imaging analyses. Application of F-ADA decreased the fluorescence ratio of the beads from the periphery to the center over 3 − 5 min. Application of F-LL also decreased the fluorescence ratio of cyLBP-cpC157 and cyLBP-cpC173 by 20−25%, and subsequent addition of IP3 recovered the fluorescence ratio in a concentration-dependent manner. The EC50 value and Hill coefficient obtained by curve fitting against the IP3-dependent recovery of fluorescence ratio can