Sandbox Reserved 1767: Difference between revisions

The presence of SHOC2 is essential for complex formation. It a crescent shaped complex that serves as a bridge for PP1C and MRAS, maximizing interaction between the three subunits of the SMP complex. SHOC2 contains a large leucine rich region (LRR) that provides stability and localizes subunit PP1C to the membrane. Houseman SHOC2 only undergoes a 6° conformational change when PP1C and MRAS bind, showing SHOC2 is a scaffolding protein that provides a favorable interface for complex formation. SHOC2 depletion is being studied as a therapeutic approach for RAS-driven cancers due to large scale interactions of the subunits only being made possible due to SHOC2. <ref name="Kwon">PMID: 35831509</ref>. SHOC2 and PP1C first engage in binding with each other via an N-terminal RVXF motif on SHOC2 that is complimentary to a sequence on PP1C. SHOC2 residues V64 and F66 embed in the complimentary region of PP1C, enhancing SHOC2 affinity for PP1C. SHOC2 bind MRAS-GTP through B strands of a LRR that interacts with a hydrophobic region of MRAS-GTP further stabilizing the complex. KWON

The Protein phosphatase complex 1 (PP1C) subunit contains the catalytic site of the SMP complex. The PP1C subunit is a phosphatase enzyme responsible for the removal of a phosphate group on the N-terminal phosphoserine (NTpS) of RAF. .<ref name="Liau">PMID: 35768504</ref> This dephosphorylation event allows for pathway activation. PP1C binds to SHOC2 and MRAS-GTP in a specific orientation that doesn’t change the conformation of the catalytic site and leaves it accessible for substrate binding. Although PP1C can dephosphorylate other proteins independently from the SMP complex, it cannot act on Raf unless bound to the complex because it lacks intrinsic substrate selectivity.<ref name="Liau">PMID: 35768504</ref> SHOC2 and MRAS aid in the specificity of the enzymatic activity. Therefore, PP1C requires the presence of SHOC2 and MRAS to function properly, whereas SHOC2 and MRAS may interact in a binary complex without the presence of PP1C <ref name="Hauseman">PMID:35830882</ref> SMP complex formation is initially mediated by SHOC2 and PP1C, then stabilized by the GTP interactions in MRAS and SHOC2.<ref name="Kwon">PMID: 35831509</ref> Similarly to SHOC2, PP1C does not undergo a significant conformational change when SHOC2 and MRAS-GTP bind. The lack of conformational change shows that the structure of PP1C is not dependent on the SMP complex, but in order to act as a phosphatase for Raf  it must be bound to the complex.<ref name="Liau">PMID: 35768504</ref>. PP1C is involved in many different cellular signaling pathways including protein synthesis, muscle contraction, and even carbohydrate metabolism. Wolfgang It plays an essential role in regulation of many pathways, not just cell proliferation. In all these pathways, including the SMP pathway, PP1C does not exist as a monomer, it is present in holoenzyme form complex with one of two regulatory subunits ensuring there is no sporadic pathway activation. Schulman

 

The active site of PP1C dephosphorylates SER259 of Raf to remove the auto-inhibition. The full mechanism for PP1C catalytic activity is unknown, however, there are 3 metal ions present (2-Mg2+  and 1 Cl-1) to stabilize the waters present in the active site. The substrate binds through hydrogen bonds with the main chain and side chain atoms of the catalytic residues **insert residue numbers here**. Mutations in the active site lead to increased activity, causing the Ras/Raf signaling cascade to be triggered more frequently.<ref name="Hurley">PMID: 17636256</ref> ***insert what residues are mutated and HOW it leads to more activity.