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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 Cys1032, His1558, and His1576 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>

[[Image:mechanismofRas.png|500 px|left|thumb|Figure 2: Mechanism of Ras Regulation by Neurofibromin. One chain of the homodimer is represented in cyan with its important domains highlighted. Ras Guanine Nucleotide Exchange Factors (RasGEFs) are shown catalyzing the transition from an active to an inactive Ras molecule and Neurofibromin is shown catalyzing the reverse reaction]]

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] (Arg1276) present in the GRD domain of Neurofibromin. Arg1276 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 Arg1276 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> The positively charged arginine finger stabilizes the transition state for GTP hydrolysis by neutralizing the negative charges on GTP, which helps increase the speed of hydrolysis, fulfilling its catalytic function. <ref name="Trovó-Marqui">PMID:16813595</ref>

Neurofibromin is a <scene name='90/904315/Homodimer/5'>homodimer</scene> 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 <scene name='90/904315/Sec14ph_and_grd_domain/1'>two important domains</scene> involved in the transition between the open and closed conformations, the <scene name='90/904316/Grd_domains/2'>GRD</scene> domain and the <scene name='90/904315/Sec14ph_domain/3'>Sec14-PH</scene> domain. The GRD and the Sec14-PH domain are centrally linked by an asymmetric, homodimeric core of four [https://en.wikipedia.org/wiki/Armadillo_repeat ARM] repeats and 27 [https://en.wikipedia.org/wiki/HEAT_repeat HEAT] repeats (Figure 3). The GRD and Sec14-PH domains extend out from the <scene name='90/904315/N-heat_arm/1'>N-HEAT/ARM</scene> core and then return to the <scene name='90/904315/C-heat_arm/1'>C-HEAT/ARM</scene> core. The orientation of the GRD and Sec-14 in relation to the N-HEAT/ARM and C-Heat/ARM determine what conformation neurofibromin is in. Although neurofibromin is a homodimer with two identical protomers, only one protomer has its GRD and Sec14-PH domains rotated into the <scene name='90/904315/Open_conformation/4'>open conformation</scene>.<ref name="Naschberger">PMID:34707296</ref>

=== Closed Conformation ===

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 Cys1032, His1558, and His1576 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> (Arg1276). 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>

Currently over 1485 mutations of <scene name='90/904315/Closed/3'>Neurofibromin</scene> have been identified that lead to life-threatening illnesses or conditions such as Neurofibromatosis Type I.<ref name="Lupton">PMID: 34887559 </ref> <ref name="Abramowicz">PMID:25182393</ref> Mutations to the NF1 gene that prevents 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. [https://en.wikipedia.org/wiki/Neurofibromatosis_type_I 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 [https://en.wikipedia.org/wiki/Caf%C3%A9_au_lait_spot cafe-au-lait macules], clusters of freckles in unusual places, and problems with the bones, eyes and nervous system.<ref name="Ratner">PMID:25877329</ref>

Neurofibromin is also involved in the differentiation of [https://en.wikipedia.org/wiki/Neural_crest neural crest-derived cells], [https://en.wikipedia.org/wiki/Mesenchymal_stem_cell mesenchymal cells], [https://en.wikipedia.org/wiki/Neural_stem_cell neural cells], [https://en.wikipedia.org/wiki/Melanocyte melanocytes], and [https://en.wikipedia.org/wiki/Osteocyte 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 to 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. <ref name="Abramowicz">PMID:25182393</ref>