Immunophilins comprise a family of proteins characterized by the presence of a specific sequence that usually shows peptidyl‐prolyl‐(cis/trans )‐isomerase (PPIase) activity, i.e., the reversible cis/trans interconversion of Xaa─Pro bonds. The “PPIase domain” also binds immunosuppressive drugs, a property used to group immunophilins into two main subfamilies: FKBPs (FK506‐binding proteins) and CyPs (cyclophilins).^[1^] Immunophilins elicited the early attention of protein folding researchers since it seemed extremely likely that they should play a cardinal role as folding catalysts.^[2^] Nonetheless, the demonstration that peptidyl‐prolyl bonds isomerization is indeed a rate‐limiting step for protein folding is still an unresolved and challenging matter. In this issue, Theo Rein addressed this conundrum by analyzing various controversial proteins such as the ribosome‐bound chaperone belonging to the FKBP subfamily, TF (trigger factor), whose PPIase activity is dispensable for nascent‐chain binding.^[3^] CyPA, a cyclophilin that induces leukocyte chemotaxis by binding to the cell surface receptor CD147, shows the same properties for PPIase‐inactive mutants. Pin1, an immunophilin upregulated in several types of cancers and downregulated in Alzheimer's disease, binds client‐proteins in a PPIase‐dependent manner. Nonetheless, it has not been demonstrated yet that cis/trans isomerization is the rate‐limiting step for this binding. An interesting case is the role of FKBP51 and FKBP52 on the glucocorticoid receptor (GR). The FD67DV mutation within the PPIase domain of FKBP51 cannot prevent the inhibitory action of this immunophilin on GR, whereas the same mutation turned FKBP52 into a GR inhibitor. This led to the conclusion that the enzymatic activity of FKBP52, but not that of FKBP51, is essential for GR regulation. However, that mutant also showed inefficient binding to client‐proteins. Consequently, other mutants were generated (Y57A, F67Y, W90L, and F130Y) showing no effect on the FKBP52‐dependent potentiation of GR. Therefore, the original concept related to the essentiality of the PPIase activity of FKBP52 on GR regulation had to be revised. Another remarkable case is the transcription factor NF‐kB, FKBP52 being a strong PPIase‐dependent enhancer and FKBP51 a PPIase‐independent inhibitory regulator in fibroblasts, but in melanoma cells, FKBP51 is a PPIase‐dependent activator. Thus, the biological action of FKBPs on NF‐kB seems to be unpredictable and cell‐type dependent. Inasmuch as immunophilins work associated to other chaperones, it is entirely possible that the final effect is dictated by different heterocomplex rearrangements or by specific protein‐protein induced conformations. An interesting speculation of Rein's hypothesis to explain these cases is that substrates could disturb protein–protein interactions due to steric interferences and/or favoring the stabilization of a given protein conformation that affects protein complex interactions. In summary, his challenging proposal asserts that the enzymatic activity of an immunophilin could be the consequence of interactions with certain protein motifs of the associated substrate, more likely through binding to peptidyl‐prolyl motifs. This is compatible with the variety of effects observed for the same client‐protein in different cell contexts discussed above and transforms the prerequisite of enzymatic activity of the immunophilin into a relative or unnecessary requirement. Therefore, the PPIase activity could be the consequence of protein interactions and not essential in most cases. An exciting possibility is that PPIase activity may have a relevant role in protein function and signal transduction rather than in protein folding per se, as it has always been thought. Of course, this hypothesis needs experimental corroboration. The use of artificial proline analogues able to impact the cis/trans preference and the energy threshold for their interconversion could be an interesting option. Also, the need of time‐resolved experiments will elucidate the relevance for accelerating prolyl‐isomerization in heterocomplexes. This is a thrilling field to develop, especially for the FKBP subfamily in view of its property to regulate signalling cascade factors, perhaps acting as transient scaffolders rather than accelerating prolyl isomerizations.

亲免素包括一个蛋白质家族，其特征在于存在特定序列，该序列通常显示肽基-脯氨酰-(顺/反)-异构酶 (PPIase) 活性，即Xaa-Pro 键的可逆顺/反相互转换。“PPIase 结构域”还与免疫抑制药物结合，这一特性用于将亲免素分为两个主要亚家族：FKBPs（FK506 结合蛋白）和 CyPs（亲环素）。^[ 1 ^]亲免蛋白引起了蛋白质折叠研究人员的早期关注，因为它们似乎极有可能作为折叠催化剂发挥重要作用。^[ 2 ^]尽管如此，肽基-脯氨酰键异构化确实是蛋白质折叠的限速步骤的证明仍然是一个未解决且具有挑战性的问题。在本期中，Theo Rein 通过分析各种有争议的蛋白质解决了这个难题，例如属于 FKBP 亚家族的核糖体结合伴侣蛋白，TF（触发因子），其 PPIase 活性对于新生链结合是不必要的。^[ 3 ^] CyPA 是一种通过与细胞表面受体 CD147 结合来诱导白细胞趋化性的亲环蛋白，对 PPIase 失活突变体显示出相同的特性。Pin1 是一种在多种癌症中上调并在阿尔茨海默病中下调的亲免素，以 PPIase 依赖性方式结合客户蛋白。尽管如此，目前还没有证明顺式/反式 异构化是这种结合的限速步骤。一个有趣的案例是 FKBP51 和 FKBP52 对糖皮质激素受体 (GR) 的作用。FKBP51 的 PPIase 结构域内的 FD67DV 突变不能阻止这种亲免素对 GR 的抑制作用，而相同的突变将 FKBP52 变成了 GR 抑制剂。这导致得出结论，即 FKBP52 的酶活性，而不是 FKBP51 的酶活性，对于 GR 调节是必不可少的。然而，该突变体也表现出与客户蛋白质的低效结合。因此，产生了其他突变体（Y57A、F67Y、W90L 和 F130Y），对 GR 的 FKBP52 依赖性增强没有影响。因此，必须修改与 FKBP52 的 PPIase 活性对 GR 调节的重要性相关的原始概念。另一个值得注意的例子是转录因子 NF-kB，FKBP52 是一种强 PPIase 依赖性增强剂，FKBP51 是成纤维细胞中 PPIase 非依赖性抑制调节剂，但在黑色素瘤细胞中，FKBP51 是 PPIase 依赖性激活剂。因此，FKBPs 对 NF-kB 的生物学作用似乎是不可预测的并且取决于细胞类型。由于亲免蛋白与其他分子伴侣相关，因此最终效果完全有可能由不同的异源复合物重排或特定的蛋白质-蛋白质诱导的构象决定。Rein 的假说解释这些情况的一个有趣的推测是，由于空间干扰和/或有利于影响蛋白质复合物相互作用的给定蛋白质构象的稳定，底物可能会干扰蛋白质 - 蛋白质相互作用。总之，他具有挑战性的提议断言，亲免素的酶活性可能是与相关底物的某些蛋白质基序相互作用的结果，更可能是通过与肽基-脯氨酰基序结合。这与在上面讨论的不同细胞环境中观察到的相同客户蛋白的各种影响相一致，并将亲免素酶活性的先决条件转化为相对或不必要的要求。因此，PPIase 活性可能是蛋白质相互作用的结果，在大多数情况下不是必需的。一个令人兴奋的可能性是，PPIase 活性可能在蛋白质功能和信号转导中具有相关作用，而不是一直认为的蛋白质折叠本身。当然，这个假设需要实验证实。顺式/反式偏好和它们相互转化的能量阈值可能是一个有趣的选择。此外，时间分辨实验的需要将阐明加速异络合物中脯氨酰异构化的相关性。这是一个令人兴奋的发展领域，特别是对于 FKBP 亚家族而言，因为它具有调节信号级联因子的特性，可能充当瞬时支架而不是加速脯氨酰异构化。