Sandbox Reserved 1790: Difference between revisions

<scene name='95/952818/Membrane/2'>MRAS</scene> is a monomeric GTPase. MRAS is membrane-bound due to post-translational lipidation, which results in lipidated residues located near the C-terminus. This lipidation allows the protein to interact with the inner membrane leaflet <ref name="Seabra">PMID:9607139</ref><ref name=”Simanshu”>PMID:28666118</ref>. MRAS localizes the SMP complex near RAF and other components of downstream signaling. The region of MRAS not directly bound to the membrane binds SHOC2 and PP1C to orient the complex such that PP1C’s active site faces the serine that will get dephosphorylated on RAF. MRAS also controls SMP complex formation in connection with extracellular signaling based on its dualistic switching between its inactive and active state. In its inactive state, MRAS is bound to GDP. When signaled by growth factors, the GDP is exchanged for GTP when a ligand binds to the RTK <ref name="Hauseman" />. The now <scene name='95/952718/Zoom_in_gtp/2'>GTP bound MRAS</scene> undergoes a conformational change of the  <scene name='95/952716/Newras-sw1-2/2'>switch I and II regions</scene>. These regions are the major binding sites with SHOC2. This conformational change activates MRAS allowing it to bind with the SHOC2-PP1C complex. In its inactive GDP-bound state, MRAS is sterically occluded from binding SHOC2. For example, R83 of GDP-bound MRAS directly clashes with SHOC2 as shown in figure 2. In comparison to other RAS proteins such as H/K/NRAS, MRAS has a greater affinity for the SHOC2-PP1C complex<ref name="Kubicek">Kubicek M, Pacher M, Abraham D, Podar K, Eulitz M, Baccarini M. Dephosphorylation of Ser-259 regulates Raf-1 membrane association. J Biol Chem. 2002 Mar 8;277(10):7913-9. [http://10.1074/jbc.M108733200 doi: 10.1074/jbc.M108733200.]</ref>. This indicates that the specific structure of MRAS is necessary for SMP function. While MRAS engages the SHOC2-PP1C complex to bring the complex to the membrane, an additional membrane-bound RAS binds RAF nearby. This binding is also stimulated by ligand binding to the RTK. This indicates that for full RAF activation and continuous signaling of Raf, two separate active RAS proteins are needed. Having two MRASs also help with the co-localization of PP1C to the NTpS region on RAF. To inactivate Raf signaling, MRAS uses its intrinsic GTPase to remove the activating gamma-phosphate on GTP. In the GDP-bound state, switch I and II move to the position shown in green in Figure 2. This inactivates SHOC2 binding due to steric clashing which causes the SMP structure to dissociate.

[[Image:Freak2.png|500 px|thumb|'''Figure 2:'''Steric clashing of Switch I and II of GDP bound MRAS, in green, with the surface of SHOC2, in magenta. GTP-bound MRAS, in white, shows no steric clashing with SHOC2s surface.</div></font>]]

[[Image:Hydro.jpg|500 px|thumb|'''Figure 1:'''Active site of PP1C on SMP.</div></font>]]

The SMP signaling pathway begins with the formation of the SMP complex. Initially, a ligand must bind to a receptor tyrosine kinase. This signals SHOC2 to bind to PP1C forming a binary complex that then binds to the membrane bound MRAS. There is some discrepancy about when the different proteins of the SMP complex come together <ref name="Liau">PMID:35768504</ref>, however we chose to depict the order ass shown in Figure 3 for a more clear visualization.  Some experiments indicate that the three proteins bind at the same time but the order is largely unknown. Once the SMP complex forms, its intracellular target is a key inactivation phosphorylation (Ser259) on MAPK Raf1. The serine is directly dephosphorylated by PP1C, while SHOC2 and MRAS increase PP1C’s specificity for S259 on Raf <ref name="Liau" />. When analyzing the surface structure of SHOC1-PP1C-MRAS, there was a hydrophobic groove on the SHOC2 terminus and another hydrophobic groove near the active site on PP1C <ref name="Liau" />. This region is crucial in making PP1C specific to Raf, because the NTpS region that is right next to the phosphoserine on Raf is able to bind to the hydrophobic patch on both SHOC2 and PP1C <ref name="Liau" />.  

Mutations affecting SMP complex formation and stability can increase or decrease MAPK signaling, where increased stability of the complex increases MAPK signaling while decreased stability decreases signaling <ref name="Liau" />. There are a set of mutations that can happen on the SMP complex as a whole that can cause [https://www.mayoclinic.org/diseases-conditions/noonan-syndrome/symptoms-causes/syc-20354422. Noonan syndrome], a rasopathy disorder <ref name="Liau" />. On SHOC2, if the following mutations S2G, C260Y, and P510L caused differences in the complex formation with PP1C <ref name="Liau" />. On PP1C, the mutation P50R resulted in stronger ionic interactions with residues on SHOC2, resulting in a more stabilized complex <ref name="Liau" />. On MRAS, mutations such as G23V and T681I, can increase the proportion of MRAS that is GTP bound, which results in increased affinity in the SMP complex overall <ref name="Liau" />. If these mutations happen all at once, or just one or two at a time, it can still significantly alter the functionality of the SMP complex <ref name="Liau" />. This can lead to diseases like Noonan syndrome, which is where there are developmental and growth issues, and can even lead to cancers <ref name="Liau" />.

[[Image:Complex.png|800 px|thumb|center|'''Figure 3:'''Signaling cascade is shown with SHOC2 (magenta), PP1C (blue), and MRAS (white). SHOC2 binds to PP1C then to MRAS at the cell membrane. The SMP complex is now oriented near the membrane bound RAF complex (green). ]]

 

[[Image:Dephosphorylate.png|900 px|thumb|center|'''Figure 4:'''PP1C dephosphorylates RAF protein at serine 259 ]]