Nucleophosmin and nucleolin regulate K-Ras signaling

Abstract

Ras proteins are laterally segregated into transient nanoclusters on the plasma membrane, a property essential for high fidelity signal transduction through the MAPK pathway. From a proteomic screen we identified nucleophosmin (NPM) and nucleolin as two novel regulators of K-Ras plasma membrane interactions that in turn influence MAP Kinase signaling. NPM and nucleolin are predominately nucleolar proteins but also possess extra-nuclear functions. We showed that a subset of NPM and nucleolin localize to the inner leaflet of the plasma membrane and specifically interact with K-Ras but not H-Ras. This interaction is independent of the activation state of K-Ras, and stabilizes K-Ras membrane levels. NPM expression also increases the fraction of K-Ras in nanoclusters. The increase in clustered K-Ras-GTP enhances signaling through the MAPK pathway. Together these results identify NPM and nucleolin as a new class of K-Ras regulators that modulate signal transduction via the MAPK pathway.

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Acknowledgements

This work was supported by award R01GM066717 from the National Institute of General Medical Sciences. J.F.H. is the current incumbent of the Fondren Chair in Cellular Signaling.

Figures and Tables

Figure 1 Model for the role of NPM, nucleolin and Gal-3 on K-Ras function. (A) Nucleolin (NCL) may operate as a chaperone to facilitate delivery of K-Ras to the plasma membrane. NPM specifically interacts with K-Ras on the plasma membrane. NCL also interacts with K-Ras on the plasma membrane probably in a complex with NPM. NPM and NCL stabilize K-Ras plasma membrane levels. This may be a consequence of increased delivery and increased nanoclustering in the case of NCL and NPM respectively. NPM alone, or NPM in complex with NCL increases the amount of K-Ras-GDP and K-Ras-GTP in nanoclusters. Increased clustering of K-Ras-GTP leads to increased ER K activation. Note that NCL and NPM are not recruited to the plasma membrane by K-ras-GTP or K-rasGDP. (B) In contrast, cytosolic Gal-3 is recruited to the plasma membrane after EGF stimulation increases K-ras-GTP levels. Recruited Gal-3 drives K-Ras-GTP nanoclustering, which leads to increased ER K activation. Whether Gal-3 nanoclusters also contain NPM and or NCL remains to be determined.

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