Inflammation, Metabolism and Oxidation Section, Department of Biomedical Sciences, University of Copenhagen, 2200 Copenhagen, Denmark.
Novo Nordisk Foundation Center for Basic Metabolic Research, University of Copenhagen, 2200 Copenhagen, Denmark.
Biomolecules. 2023 Jan 11;13(1):152. doi: 10.3390/biom13010152.
Apart from chaperoning, disulfide bond formation, and downstream processing, the molecular sequence of proinsulin folding is not completely understood. Proinsulin requires proline isomerization for correct folding. Since FK506-binding protein 2 (FKBP2) is an ER-resident proline isomerase, we hypothesized that FKBP2 contributes to proinsulin folding. We found that FKBP2 co-immunoprecipitated with proinsulin and its chaperone GRP94 and that inhibition of FKBP2 expression increased proinsulin turnover with reduced intracellular proinsulin and insulin levels. This phenotype was accompanied by an increased proinsulin secretion and the formation of proinsulin high-molecular-weight complexes, a sign of proinsulin misfolding. FKBP2 knockout in pancreatic β-cells increased apoptosis without detectable up-regulation of ER stress response genes. Interestingly, FKBP2 mRNA was overexpressed in β-cells from pancreatic islets of T2D patients. Based on molecular modeling and an in vitro enzymatic assay, we suggest that proline at position 28 of the proinsulin B-chain (P28) is the substrate of FKBP2's isomerization activity. We propose that this isomerization step catalyzed by FKBP2 is an essential sequence required for correct proinsulin folding.
除了伴侣、二硫键形成和下游加工外，胰岛素原折叠的分子序列还不完全清楚。胰岛素原需要脯氨酸异构化才能正确折叠。由于 FK506 结合蛋白 2（FKBP2）是内质网驻留的脯氨酸异构酶，我们假设 FKBP2 有助于胰岛素原折叠。我们发现 FKBP2 与胰岛素原及其伴侣 GRP94 共免疫沉淀，并且 FKBP2 表达的抑制增加了胰岛素原的周转率，导致细胞内胰岛素原和胰岛素水平降低。这种表型伴随着胰岛素原分泌增加和胰岛素原高分子量复合物的形成，这是胰岛素原错误折叠的标志。在胰岛β细胞中敲除 FKBP2 会增加细胞凋亡，而内质网应激反应基因没有明显上调。有趣的是，T2D 患者胰岛β细胞中的 FKBP2 mRNA 表达过度。基于分子建模和体外酶促测定，我们认为胰岛素原 B 链第 28 位的脯氨酸（P28）是 FKBP2 异构化活性的底物。我们提出，FKBP2 催化的这种异构化步骤是胰岛素原正确折叠所必需的序列。
