Insights into the oncogenic effects of PIK3CA mutations from the structure of p110alpha/p85alpha

Abstract

Phosphatidylinositide-3-kinases (PI3K) initiate a number of signaling pathways by recruiting other kinases, such as Akt, to the plasma membrane. One of the isoforms, PI3Kalpha, is an oncogene frequently mutated in several cancer types. These mutations increase PI3K kinase activity, leading to increased cell survival, cell motility, cell metabolism, and cell cycle progression. The structure of the complex between the catalytic subunit of PI3Kalpha, p110alpha, and a portion of its regulatory subunit, p85alpha reveals that the majority of the oncogenic mutations occur at the interfaces between p110 domains and between p110 and p85 domains. At these positions, mutations disrupt interactions resulting in changes in the kinase domain that may increase enzymatic activity. The structure also suggests that interaction with the membrane is mediated by one of the p85 domains (iSH2). These findings may provide novel structural loci for the design of new anti-cancer drugs.

Fig. 1. Ribbon diagram of the p110α/niSH2 heterodimer

(A) Scheme of the domain organization. The same color coding is used throughout this article unless specified. Gray regions are linkers between domains. The ABD domain of p110α is green; RBD is orange; C2 is blue; helicase is red; kinase is purple. The iSH2 domain of p85α is colored yellow. The nSH2 domain of p85α (not shown; colored as a rainbow in the scheme of the construct), was not traced but it was modeled into weak density using the deposited coordinates.

Fig. 2. Interaction between the iSH2 domain of p85 and the ABD and the C2 domain of p110

(A) Molecular surface of the p110α /niSH2. The iSH2 is shown as a molecular surface colored according to electrostatic potential. The ABD and the C2 domain are shown as ribbons. (b) The same image rotated by 90°. (C) Close-up of the interaction between iSH2 and the ABD. (D) Close-up of the interaction between iSH2 and the C2 domain.

Fig. 3. Intermolecular contact in the p110α /p85 heterodimer crystal

(A) Molecular surface of the PI3K colored as electrostatic charges showing the Ras binding domain of a neighboring molecule in the crystal (orange ribbon with black back-side) bound in the kinase domain active site. (B) Molecular surface colored as electrostatic potential showing the ATP (ball-and-stick representation) bound to PI3Kα . (C) Molecular surface colored according to the electrostatic potential showing the helix-loop-helix motif of the RBD of the neighboring molecule in the ATP binding site.

Fig. 4. Mutations in PIK3CA identified in human cancers

(A) Location of representative mutations within p110α. Residues mutated in cancers are shown as CPK models. The start of the cancer associated truncation (residue 571 of p85) is shown by the green arrowhead. (B) Residues Arg38 and Arg88 , frequently mutated in cancers, are shown at the interface between the ABD and the kinase domains. (C) Contacts between the C2 and iSH2 domain in the p110α /p85 heterodimer. Asn345 of C2 and the residues within iSH2 (Asp560 and Asn564) with which it may interact are shown with a stick representation. (D) Residues in the helical domain commonly mutated in cancers (Glu542, Glu545, and Gln 546) are located at the interface with nSH2 (grey surface) in close proximity to the nSH2-kinase domain interface. (E) Mutations of the kinase domain, (Met 1043 and His1047), located near the C-terminal end of the activation loop, are shown in yellow. The portion of the activation loop between residues 941 to 950, not traced in the published structure, is shown as a dashed line.