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In Kessler et al. (1), the small molecule BI-2852 is shown to bind—at nanomolar affinity—to KRAS between switch I and II, inhibiting interactions with effectors. Here, we identify an alternative explanation for the inhibitory activity. While investigating the BI-2852 KRAS complex (Protein Data Bank [PDB] code 6GJ8), we noticed, by revealing the molecules in the neighboring unit cell, BI-2852 induces a KRAS dimer with rotational symmetry. This dimer complex consists of four molecules (two of BI-2852 and two of KRAS; Fig. 1 A ). Fig. 1. Dimer of KRAS with BI-2852. ( A ) Cartoon of BI-2852 KRAS dimer. Lig1 (cyan) and Lig2 (magenta) interact with both Ras1 (beige) and Ras2 (orange). ( B ) KRAS (ribbons) and key side chains are shown. ( C ) Zoom-in of the binding site: salt–bridge interactions shown (dashed lines). ( D ) Lid (green) and Basin (purple) are indicated on Ras1. Both Lig1 and Lig2 are shown on Ras1. The indole rings, colored green and purple, engage the Lid and Basin, respectively. ( E ) Absorbance A 280 plotted as a function of elution volume: … [↵][1]1To whom correspondence may be addressed. Email: trent.balius{at}nih.gov. [1]: #xref-corresp-1-1

Farnesylation and carboxymethylation of KRAS4b (Kirsten rat sarcoma isoform 4b) are essential for its interaction with the plasma membrane where KRAS-mediated signaling events occur. Phosphodiesterase-δ (PDEδ) binds to KRAS4b and plays an important role in targeting it to cellular membranes. We solved structures of human farnesylated-methylated KRAS4b in complex with PDEδ in two different crystal forms. In these structures, the interaction is driven by the C-terminal amino acids together with the farnesylated and methylated C185 of KRAS4b that binds tightly in the central hydrophobic pocket present in PDEδ. In crystal form II, we see the full-length structure of farnesylated-methylated KRAS4b, including the hypervariable region. Crystal form I reveals structural details of farnesylated-methylated KRAS4b binding to PDEδ, and crystal form II suggests the potential binding mode of geranylgeranylated-methylated KRAS4b to PDEδ. We identified a 5-aa-long sequence motif (Lys-Ser-Lys-Thr-Lys) in KRAS4b that may enable PDEδ to bind both forms of prenylated KRAS4b. Structure and sequence analysis of various prenylated proteins that have been previously tested for binding to PDEδ provides a rationale for why some prenylated proteins, such as KRAS4a, RalA, RalB, and Rac1, do not bind to PDEδ. Comparison of all four available structures of PDEδ complexed with various prenylated proteins/peptides shows the presence of additional interactions due to a larger protein-protein interaction interface in KRAS4b-PDEδ complex. This interface might be exploited for designing an inhibitor with minimal off-target effects.