Regulation of K-RAS Membrane Association: Calmodulin Versus PDEδ

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K-Ras is a small GTPase that plays a critical role in human cancer cell biology. Selective membrane localization and clustering of K-Ras4B into microdomains are mediated by its polybasic farnesylated C-terminus. The importance of the subcellular distribution for the signaling activity of K-Ras4B became apparent from recent in vivo studies [1]. PDEδ and the Ca2+-binding protein calmodulin (CaM) are known to function as potential binding partners for farnesylated Ras proteins, leading to a modulation of the spatiotemporal organization of K-Ras4B. The latest study of our group showed that PDEδ is not able to extract K-Ras4B from model raft membranes; instead, an effective delivery of PDEδ-solubilized K-Ras4B to the plasma membrane was proposed [2]. Compared to PDEδ, CaM exhibits additional interaction sites to the G-domain of K-Ras4B and was shown not to be required for the transport of K-Ras4B to the plasma membrane. Thus, it was suggested that calmodulin dissociates K-Ras4B from membranes [3]. However, the exact role of CaM in the intracellular localization and dynamics of K-Ras4B still remains elusive.In the present approach, the influence of CaM on the interaction of GDP- and GTP-loaded K-Ras4B with anionic model raft membranes has been investigated by a combination of different spectroscopic and imaging techniques. The results suggest that binding of the acidic CaM to the polybasic stretch of K-Ras4B reverses its charge, leading to repulsion of the complex from anionic membranes. Since one farnesyl anchor alone is not sufficient to stably anchor Ras proteins to membranes, CaM would be able to dissociate K-Ras4B from plasma membranes, contrary to PDEδ.