User:Peyton Jenkins/Sandbox 1

Relevance and Disease

Lung cancer is the leading cause of cancer related death worldwide. In the United States alone, over 120,000 deaths were caused by lung cancer in 2024^[1]. Non small cell lung cancer make up approximately 84% of all lung cancer cases, and of these lung adenocarcinoma accounts for about 65%^[2]. In lung adenocarcinoma, STK11 is the third most commonly mutated gene, behind only KRAS and p53^[3].

Germline loss of function mutations in STK11 are associated with Peutz-Jeghers syndrome. A precancerous condition characterized by the formation of polyps in the small intestine, and a predisposition to all cancers.

Serine/Threonine Kinase 11 (STK11) is a master kinase, signalling upstream of the AMP-Activated Protein Kinase (AMPK) family, p53, and Focal Adhesion Kinase (FAK), to regulate processes like anoikis, adhesion, growth, metabolism, and survival^{[4], [5]}. STK11 exists in a with the pseudokinase STE Related Adaptor Alpha (STRADα), and the scaffolding protein Mouse Protein 25 (MO25). Unlike other kinases that are activated by phosphorylation within the activation loop, STK11 is activated by the formation of this complex and thus it is essential for both proper kinase activity and proper localization. ^{[6], [7]}

Structural Highlights

STK11

STK11 can be broken down into 3 domains. An N-terminal domain (aa 1-42), kinase domain (aa 43-347), and a C-terminal domain (aa 348-433). The of STK11 is located from residues ~202-212. Within the activation loop is F204, which interacts with a hydrophobic pocket on MO25, which is necessary to stabilize the active conformation. Within the αC helix is residue D98 which forms a with K78, further stabilizing the active site and aids in ATP binding. Mg²⁺ helps aid ATP binding, however in this structure there is a point mutation () to make STK11 catalytically inactive. Another loop, the is essential for binding of STRADα on the N-terminal lobe of STK11. β7-β8 sheets of STK11 also interact with STRADα. In the β2-β3 loop R74 hydrogen bonds with Q251 of STRADα.

STRADα

STRAD alpha is composed of 2 domains, an N-terminal domain (aa 1-58) and pseudokinase domain (aa 59-431). STRADα is termed a pseudokinase because it shares structural features, such as an activation loop and αC helix, with other kinases, but lacks catalytic activity. STRADα binds STK11 through its pseudokinase domain, with the interacting with the the β2-β3 loop and β7-β8 sheets of STK11. The interacts with the surface of MO25, further stabilizing the interaction between proteins. Additionally there is a (aa 429-431) on the C-terminus of STRADα interacting with the C-terminus of MO25.

MO25

MO25 is a repeat of α-helices spanning the entire length of the protein. Residues interact with the A205 and A206 of the STK11 activation loop. This is not required for MO25 and STK11 binding, however mutating R240 and F243 resulted in a catalytically inactive complex, thus these residues are essential to orient and stabilize the active conformation of STK11.

Missing Residues

This structure is missing important residues both on STK11 and STRADα. In STRADα, the last portion of the pseudokinase domain is also missing, aside from the WEF motif. This region is important for MO25 binding. From STK11, both the N-terminal domain and C-terminal domain are almost entirely missing. Despite not being essential for catalytic activity, these domains have important kinase-independent function. The N-terminal domain has been shown to negatively regulate FAK, and loss of FAK regulation leads to altered cell motility. The C-terminal domain has been shown to be important for development in Drosophila through interactions with phosphatidic acid along the plasma membrane, and also contains a farnesylation site to aid in intracellular localization. The C-terminal domain has also been shown to regulate RhoA and cdc42 and affect the phenotype of invasion in a 3D cell culture model.

Pathogenic Mutations

When a patient presents with a mutation to STK11, the gold standard for determining if this mutation is pathogenic is to compare it to a list of known Peutz-Jegher mutants, as this condition is known to be caused by loss of function in STK11. If a mutation is not present in Peutz-Jegher Syndrome, it is referred to as a "variant of unknown significance", and further testing must be done in order to classify these mutants as either pathogenic, or benign. The first step in this is to determine if the mutant retains kinase activity

References

1. ↑ https://www.cancer.org/content/dam/cancer-org/research/cancer-facts-and-statistics/annual-cancer-facts-and-figures/2024/2024-cancer-facts-and-figures-acs.pdf
2. ↑ 10.1001/jamaoncol.2021.4932
3. ↑ 10.1091/mbc.E15-08-0569.
4. ↑ 10.1038/sj.emboj.7600110
5. ↑ 10.1074/jbc.M112.444620
6. ↑ 10.1038/sj.emboj.7600110
7. ↑ 10.1093/emboj/cdg490