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Neurofibromin functions as a tumor suppressor protein by associating with [https://proteopedia.org/wiki/index.php/Ras Ras], a protein that signals for [https://en.wikipedia.org/wiki/Cell_proliferation cell proliferation] and growth. It has two conformations, the open, active conformation and the closed, inactive conformation (Figure 1). <ref name="Trovó-Marqui">PMID:16813595</ref> The change in conformation of Neurofibromin from open to closed regulates Ras activity, turning it off, thus stopping cell growth from continuing in excess. Neurofibromin is a [https://en.wikipedia.org/wiki/Dimer_(chemistry) dimer] made up of two identical [https://en.wikipedia.org/wiki/Monomer monomers] consisting of the Sec 14-PH domain and the GRD (Gap-related domain). The orientation of these domains is important for the interaction and binding of Ras to the Arginine finger in Neurofibromin, a key interaction in the regulation of Ras activity. Neurofibromin in its open conformation allows for the Ras association, unlike the closed conformation which is unable to associate with Ras and turn it off. <ref name="Naschberger">PMID:34707296</ref> The closed conformation is stabilized by a triad consisting of Cys 1032, His 1558, and His 1576 and a Zinc atom. The molecular structure and detail of Neurofibromin have been determined by [https://en.wikipedia.org/wiki/Cryogenic_electron_microscopy Cryo-Electron Microscopy]. Additionally, the structure of Neurofibromin isoform 2 revealed different functional states for Neurofibromin. <ref name="Naschberger">PMID:34707296</ref> Neurofibromin is encoded by the [https://en.wikipedia.org/wiki/Neurofibromin_1 NF1 gene], which is located on chromosome 17. Mutations in the NF1 gene are associated with diseases including Neurofibromatosis Type 1, [https://en.wikipedia.org/wiki/Neurofibroma Plexiform Neurofibromas], and cancers including glioblastoma, neuroblastoma, lung, ovarian, and breast cancer. <ref name="Lupton">PMID: 34887559 </ref> <ref name="Ratner">PMID:25877329</ref> <ref name="Abramowicz">PMID:25182393</ref>

Neurofibromin functions as a tumor suppressor protein.<ref name="Trovó-Marqui">PMID:16813595</ref> It prevents cell growth by turning off [https://en.wikipedia.org/wiki/Ras_GTPase Ras] which in its active state, stimulates cell growth and division (Figure 2). Ras is a small, monomeric [https://en.wikipedia.org/wiki/GTPase GTPase] that binds to [https://en.wikipedia.org/wiki/Guanosine_triphosphate GTP] and hydrolyzes it to [https://en.wikipedia.org/wiki/Guanosine_diphosphate GDP]. When Ras is active, it subsequently activates other proteins that stimulate cell growth and proliferation. Ras is membrane-bound and interacts with Neurofibromin, a cytoplasmic protein, in the open conformation of Neurofibromin. Neurofibromin is brought to the membrane to associate with Ras by [https://en.wikipedia.org/wiki/SPRED1 SPRED1]. [https://medlineplus.gov/genetics/gene/spred1/ SPRED1] is a protein that helps regulate the [https://en.wikipedia.org/wiki/MAPK/ERK_pathway Ras/MAPK signaling pathway] responsible for the growth and proliferation of cells. Binding of SPRED1 to [https://en.wikipedia.org/wiki/RAF_kinase Raf] in the MAPK signaling pathway blocks the activation of Raf, halts the rest of the pathway, and stops cell growth and proliferation. Unlike Ras, Neurofibromin can interact with SPRED1 in both the open and closed conformations.<ref name="Naschberger">PMID:34707296</ref> The interaction between Neurofibromin and Ras is activated via an [https://en.wikipedia.org/wiki/Arginine_finger 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. Ras binds to the arginine finger of Neurofibromin with its switch regions 1 and 2.<ref name="Trovó-Marqui">PMID:16813595</ref When Arg 1276 is able to associate with Ras, Neurofibromin downregulates the [https://en.wikipedia.org/wiki/MAPK/ERK_pathway 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 is inhibited. <ref name="Naschberger">PMID:34707296</ref>  

In the <scene name='90/904315/Closed/3'>closed, inactive conformation</scene>, 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 <scene name='90/904315/Closed_triade/7'>triad</scene> consisting of residues Cys 1032, His 1558, and His 1576 that form a transition metal-binding site with zinc. The rigid organization of the <scene name='90/904315/Closed_triade/8'>triad in closed conformation</scene> keeps the GRD domain packed tightly on top of the Heat Arms in the Neurofibromin core. This tight compaction sterically occludes Neurofibromin from <scene name='90/904315/Closed_with_ras/2'>associating with Ras.</scene> In its active form, Ras and Neurofibromin will associate via an <scene name='90/904315/Arg_finger/5'>Arginine Finger</scene>(Arg 1276). However, the <scene name='90/904315/Arg_finger/3'>steric hindrance</scene> 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.<ref name="Naschberger">PMID:34707296</ref>

In the <scene name='90/904315/Open_conformation/4'>open, active conformation</scene>, the GRD and Sec14-PH domain on one protomer have [https://youtu.be/I1I4uTVFR00 rotated] and become accessible for binding to Ras. This transition is initiated by the 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 (Figure 4). 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 <scene name='90/904315/Open_conformation_triade/6'>triad</scene>. The lack of the transition metal-binding site allows the GRD to orient itself to <scene name='90/904315/Openwithras/1'>associate with Ras</scene><ref name="Naschberger">PMID:34707296</ref>. This association positions the <scene name='90/904315/Open_conformation_arginine_fin/3'>Arginine Finger</scene> to help stabilize and orient a catalytic Ras residue (Q61) so that the gamma phosphate of GTP can be nucleophilically attacked <ref>PMID:33121128</ref>. When Neurofibromin is in the open, active conformation, <scene name='90/904315/Open_conformation_arginine_fin/4'>Arg1276</scene> is able to bind to Ras because there is no steric hindrance from the Neurofibromin core.<ref name="Naschberger">PMID:34707296</ref>