Sandbox Reserved 1710

This Sandbox is Reserved from February 28 through September 1, 2022 for use in the course CH462 Biochemistry II taught by R. Jeremy Johnson at the Butler University, Indianapolis, USA. This reservation includes Sandbox Reserved 1700 through Sandbox Reserved 1729.

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Human Neurofibromin - The Tumor Suppressor Gene

spinqualitylabelsall models Neurofibromin in the Closed Conformation (7PGR). The identical chains that make up the dimer are colored lime and cyan. Two important domains in the function of Neurofibromin are highlighted with the GRD colored red and the Sec14-PH domain colored magenta. Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image   Print3D Introduction Figure 1: Surface Rendering of Neurofibromin in its Open (7PGT) and Closed (7PGR) Conformation. Neurofibromin is encoded by NF1 gene, located on chromosome 17. Neurofibromin functions as a tumor suppressor through its association with the protein Ras. The molecular structure of Neurofibromin has been determined by Cryo-Electron Microscopy. The structure of neurofibromin isoform 2 revealed different functional states for the Neurofibromin protein. Mutations in Neurofibromin are associated with diseases such as Plexiform Neurofibromas. (FLESH OUT INTRODUCTION) Function Figure 2: Neurofibromin Important Domains in one chain of the homodimer Neurofibromin functions as a tumor suppressor protein. It prevents cell growth by turning off Ras which in its active state, stimulates cell growth and division. Ras is a small, monomeric GTPase. Ras is membrane-bound and interacts with Neurofibromin, a cytoplasmic protein, in the open conformation of Neurofibromin. (DEFINE RAS) Neurofibromin is brought to the membrane to associate with Ras by SPRED1. (DEFINE SPRED1) Unlike Ras, Neurofibromin can interact with SPRED1 in both the open and closed conformations. The interaction between Neurofibromin and Ras is activated via an Arginine finger (Arg 1276) present in the GRD domain of Neurofibromin. Arg 1276 is only accessible for binding when the GRD and Sec14-PH domains are rotated into the open conformation. When Arg 1276 is able to associate with Ras, Neurofibromin downregulates the Ras signaling pathway by speeding up Ras's GTPase activity, hydrolyzing the GTP associated with Ras to GDP. In its GDP bound state, Ras is inactive and cell growth and division are inhibited. (MECHANISTIC EXPLANATION). Mechanism of Ras Regulation by Neurofibromin Structure Neurofibromin is a homodimer made up of two identical chains. Neurofibromin has two conformations, open and closed. Shifting between these controls neurofibromin's ability to associate with Ras and perform its function of Ras regulation. The transformation between the overall closed and open conformations transitions it from an active to inactive state. There are two important domains involved in the transition between the open and closed conformations, the GRD domain and the Sec14-PH domain. Although neurofibromin is a homodimer with two identical protomers, only one protomer has its GRD and Sec14-PH domains rotated into the open conformation. N-HEAT/ARM C-HEAT/ARM Conformational States Closed Conformation In the closed, inactive conformation, the GRD and Sec14-PH domains are rotated so Ras cannot bind. In this conformation, the GRD and Sec14-PH are inaccessible and inactive. Neurofibromin is held in the inactive state by a triad consisting of residues Cys 1032, His 1558, and His 1576 that form a transition metal-binding site with zinc. The rigid organization of the triad in closed conformation keeps the GRD domain packed tightly on top of the Heat Arms in the Neurofibromin core. This tight compaction sterically occludes Neurofibromin from associating with Ras. In its active form, Ras and Neurofibromin will associate via an Arginine finger (Arg 1276). However, the steric hindrance from the Neurofibromin core in the closed conformation inhibits this association. Therefore, in the closed conformation, neurofibromin cannot catalyze GTP hydrolysis by Ras and Ras continues to signal for cell growth and proliferation. Open Conformation In the open, active conformation, (REWRITE ABOUT RAS REGULATION) This transition is initiated by movement of the transition metal-binding site. The Cys 1032, His 1558, and His 1576 Residues become separated and zinc is not able to bond. In the active form, one protomer has its GRD and Sec14-PH domains oriented oppositely from the inactive form. The GRD rotates -130° and the Sec14-PH domain rotates -90° away from the N-HEAT/ARM. Due to this rotation, Cys 1032 is now located too far away, approximately 30 Å, from His 1558 and His 1576 which results in the loss of the metal-binding site and no formation of the triad. The lack of the transition metal-binding site allows the GRD to orient itself to associate with Ras. This association positions the Arginine Finger (TO DO WHAT?). When Neurofibromin is in the open, active conformation, Arg 1276 is able to bind to Ras because there is no steric hindrance from the Neurofibromin core. Figure 3: Rotation of the GRD and Sec14-PH domains from the closed conformation (7PGR) of neurofibromin to the open conformation (7PGT) of neurofibromin to allow Ras binding. The GRD rotates -130° and the Sec14-PH domain rotates -90° Disease and Medical Relevance Currently over 1485 mutations of Neurofibromin have been identified that lead to life-threatening illnesses or conditions such as Neurofibromatosis Type I. Mutations to the NF1 gene that prevent the interaction of Neurofibromin with Ras, remove this check on Ras-dependent cell proliferation and uncontrolled cell growth can lead to tumors and a higher risk of cancer. Neurofibromatosis Type 1, the most well-known disease resulting from mutations in Neurofibromin, is characterized by cognitive impairment, soft, non-cancerous tumors on or under the skin known as neurofibromas, birthmarks called cafe-au-lait macules, clusters of freckles in unusual places, and problems with the bones, eyes and nervous system. Neurofibromin is also involved in the differentiation of neural crest-derived cells, mesenchymal cells, neural cells, melanocytes, and bone cells. As Neurofibromin is essential for embryonic development, mutations to the NF1 gene can result in psychological retardation resulting from Type I neurofibromatosis. Most of the 1485 point mutations identified lead to a synthesis of truncated, non-functional protein. Type I Neurofibromatosis is inherited in an autosomal dominant manner but about 50% of cases result from de novo mutations. Mutations in the NF1 gene have been found in sporadic cancers such as glioblastoma, neuroblastoma, lung cancer, ovarian cancer, and breast cancer. Researchers are still unsure as to whether biallelic loss of NF1 is common or if it is only a hemizygous loss of the gene that contributes to the growth progression of certain sporadic tumors. Student Contributors Hannah Luchinski Sophie Mullinix