Part 1: Summary
Signaling pathway is a series of biochemical events initiated, after a cell receives extraneous stimuli, by factors such as hormones. The activation of the adapter proteins turns on the connections among the pathways and allows the signal transfer (Flynn). The adaptor protein Crk, for example, influences the physiology of chronic myelogenous leukemia (CML). Many proteins regulate the off and on states by shifting between the trans and cis configurations of a proline switch. In Crk, the cis-trans isomerization modulates the proline switch. The switch is off in the cis state, and the Crk protein is inactive. The switch, however, is on in the trans state, and the Crk is active. Sarkar et al. found that the three domains of Crk, SH2, SH3N, and SH3C, were significant in the mediation of the off and on states of the proline switch. They demonstrated that the region of amino acids, the linker, between the SH3C and SH3N contains the proline switch. They hypothesized that in the trans conformation, the Crk protein was active because the residues in the SH3C field and the proline switch had a complete solvent exposure. The understanding of the Crk activation mechanism is crucial to the development of effective drugs for fighting diseases such as the CML. The intracellular enzyme Cyp A, which accelerates the switch between the two configurations, is also a potential target for the development of drugs.
Part 2: Discussion
The proline cis-trans isomerization is a regulatory mechanism that regulates the activity of various bio-molecular processes. The structural data presented by Sarkar et al. demonstrates that the proline isomerization results in the restructuring of the interface between the SH3C field and the amino acids’ region. The two resultant conformers have discrete binding properties. The trans conformer promotes an uninhibited and dumbbell-like conformation while the cis promotes the intramolecular involvement of the SH3 domains in setting up an autoregulatory conformation. In the Crk protein, the Pro238 occurs at the connection between the SH3C domain and the region of amino acids. The linker interacts widely with the SH3C field in the trans conformer. At the Pro238, the trans-to-cis conversion forces a new configuration for the linker and disrupts the contacts, between the SH3C field and the linker, present in the trans structure. In the trans structure, the dumbbell-like conformation occurs because the SH3N-SH3C interaction does not take place. Since the proline switch in the Crk toggles the protein between an open and a closed conformation, it has a crucial role in the physiological process. The proline isomerization is a slow process that reduces the speed of the molecular switch kinetics. The cyclophilin A (Cyp A), however, increases the rate of the inter-conversion at physiological temperatures. The proline isomerization in Crk, therefore, works as a molecular timer that controls the rate of formation of a signaling complex. Sarkar et al. speculate that proline isomerization might be a conserved aspect of Crk because all Crk proteins conserve the Pro238. Their results also reveal that the residue’s identity, following the isomerization, influences the cis population. The results obtained by Sarkar et al. highlight the significance of the SH3C field in stabilizing and establishing the autoinhibitory configuration in Crk. The point deletions and mutations in the SH3C field accelerate the transforming activity of the Crk and raise its expression levels in lung tumors and malignant brains. The interaction between the SH3C and SH3N illustrates the mediation of an SH3-SH3 complex by the direct contact of the two domains.
Conclusion
Proline switches are useful in regulating the activities of many biological processes. The cis-trans isomerization controls the proline switches. The isomerization of Proline toggles the Crk between uninhibited and autoinhibitory conformations. The trans structure stabilizes the uninhibited and activated configuration while the association of two SH3 fields in the cis form stabilize the autoinhibitory configuration. Apart from its role as a switch, the proline uses cyclophilin A to speed up the interconversion rate between the isomers. As a result, it controls the activation kinetics of the Crk protein.
Works Cited
Flynn, Daniel C. “Adaptor proteins.” Oncogene 20.44 (2001): 6270-6272. Web. 21 June 2014.
Sarkar, Paramita, Tamjeed Saleh, Shiou-Ru Tzeng, Raymond B.Birge, and Charalampos G. Kalodimos. “Structural basis for the regulation of the Crk signaling protein by a proline switch.” Nature Chemical Biology 7 (2011): 51–57. Web. 21 June 2014.
