Abstract
KRAS lipid specificity controls signaling and is not determined solely by electrostatic attraction.
Major finding: KRAS lipid specificity controls signaling and is not determined solely by electrostatic attraction.
Concept: KRAS mutants have distinct lipid specificities resulting in nanoclusters with various lipid compositions.
Impact: Small changes in the KRAS anchor sequence can alter lipid specificity to modulate KRAS activity.
KRAS is tethered to the plasma membrane via a C-terminal membrane anchor comprised of an S-farnesyl cysteine carboxylmethyl ester and a polybasic domain (PBD) of six contiguous lysines. KRAS laterally diffuses in the plasma membrane to form nanoclusters containing approximately six KRAS proteins that serve as platforms for effector binding and signal transduction. Many other proteins contain PBDs, and it has been assumed that they all mediate nonspecific electrostatic interactions with anionic phospholipids of the plasma membrane. However, KRAS nanoclusters are enriched for monovalent phosphatidylserine and phosphatidic acid, and contain little multivalent phosphoinosital 4,5-bisphosphate (PIP2), prompting Zhou and colleagues to hypothesize that KRAS membrane anchors might interact with specific anionic lipids beyond recognition of the charged lipid head group. Consistent with this hypothesis, KRAS nanocluster assembly at the plasma membrane required asymmetric phosphatidylserine. Altering the membrane anchor by replacing the six PBD lysines with arginines, which have the same net charge, replacing farnesylated KRAS with geranylated KRAS, or both, resulting in distinct lipid compositions in the KRAS nanoclusters, indicating a lipid specificity independent of charge. Further, wild-type KRAS had a different lipid composition from constitutively active KRASG12V. Computational analyses revealed that the structure of the KRAS lipid anchor is affected by interactions with specific lipids, not based solely on electrostatic attraction, and is thus affected by mutations or prenylation. Moreover, mutations in the PBD sequence altered lipid binding specificity and plasma membrane nanoclustering, which led to changes in activation of downstream signaling. Collectively, these findings indicate that the KRAS membrane anchor binds specifically to plasma membrane lipids to form nanoclusters with defined lipid compositions that determine KRAS signaling output.
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