fully-processed-recombinant-kras4b-isolating-characterizing

Fully Processed Recombinant KRAS4b: Isolating and Characterizing the Farnesylated and Methylated Protein

Protein prenylation is a key modification that is responsible for targeting proteins to intracellular membranes. KRAS4b, which is mutated in 22% of human ...

Frederick National Laboratory for Cancer Research

Deregulation of KRAS4b signaling pathway has been implicated in 30% of all cancers. Membrane localization of KRAS4b is an essential step for the initiation of the downstream signaling cascades that guide various cellular mechanisms. KRAS4b plasma membrane (PM) binding is mediated by the insertion of a prenylated moiety that is attached to the terminal carboxy-methylated cysteine, in addition to electrostatic interactions of its positively charged hypervariable region with anionic lipids. Calmodulin (CaM) has been suggested to selectively bind KRAS4b to act as a negative regulator of the RAS/mitogen-activated protein kinase (MAPK) signaling pathway by displacing KRAS4b from the membrane. However, the mechanism by which CaM can recognize and displace KRAS4b from the membrane is not well understood. In this study, we employed biophysical and structural techniques to characterize this mechanism in detail. We show that KRAS4b prenylation is required for binding to CaM and that the hydrophobic pockets of CaM can accommodate the prenylated region of KRAS4b, which might represent a novel CaM-binding motif. Remarkably, prenylated KRAS4b forms a 2:1 stoichiometric complex with CaM in a nucleotide-independent manner. The interaction between prenylated KRAS4b and CaM is enthalpically driven, and electrostatic interactions also contribute to the formation of the complex. The prenylated KRAS4b terminal KSKTKC-farnesylation and carboxy-methylation is sufficient for binding and defines the minimal CaM-binding motif. This is the same region implicated in membrane and phosphodiesterase6-δ binding. Finally, we provide a structure-based docking model by which CaM binds to prenylated KRAS4b. Our data provide new insights into the KRAS4b-CaM interaction and suggest a possible mechanism whereby CaM can regulate KRAS4b membrane localization.

KRAS Prenylation Is Required for Bivalent Binding with Calmodulin in a Nucleotide-Independent Manner.

Although post-translational modification of the C-terminus of RAS has been studied extensively, little is known about N-terminal processing. Mass spectrometric characterization of KRAS expressed in mammalian cells showed cleavage of the initiator methionine (iMet) and N-acetylation of the nascent N-terminus. Interestingly, structural studies on GDP- and GMPPNP-bound KRAS lacking the iMet and N-acetylation resulted in Mg-free structures of KRAS with flexible N-termini. In the Mg-free KRAS-GDP structure, the flexible N-terminus causes conformational changes in the interswitch region resulting in a fully open conformation of switch I. In the Mg-free KRAS-GMPPNP structure, the flexible N-terminus causes conformational changes around residue A59 resulting in the loss of Mg and switch I in the inactive state 1 conformation. Structural studies on N-acetylated KRAS-GDP lacking the iMet revealed the presence of Mg and a conformation of switch regions also observed in the structure of GDP-bound unprocessed KRAS with the iMet. In the absence of the iMet, the N-acetyl group interacts with the central beta-sheet and stabilizes the N-terminus and the switch regions. These results suggest there is crosstalk between the N-terminus and the Mg binding site, and that N-acetylation plays an important role by stabilizing the N-terminus of RAS upon excision of the iMet.

Constance Agamasu

NCI RAS Initiative,
Cancer Research Technology Program,
NCI RAS Initiative, Cancer Research Technology Program

Frederick National Laboratory for Cancer Research

Fully Processed Recombinant KRAS4b: Isolating and Characterizing the Farnesylated and Methylated Protein

Constance Agamasu

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Frederick National Laboratory for Cancer Research

Fully Processed Recombinant KRAS4b: Isolating and Characterizing the Farnesylated and Methylated Protein

NCI RAS Initiative,
Cancer Research Technology Program,
NCI RAS Initiative, Cancer Research Technology Program