The regulation of class IA PI 3-kinases by inter-subunit interactions

Abstract

Phosphoinositide 3-kinases (PI 3-kinases) are activated by growth factor and hormone receptors, and regulate cell growth, survival, motility, and responses to changes in nutritional conditions (Engelman et al. 2006). PI 3-kinases have been classified according to their subunit composition and their substrate specificity for phosphoinositides (Vanhaesebroeck et al. 2001). The class IA PI 3-kinase is a heterodimer consisting of one regulatory subunit (p85α, p85β, p55α, p50α, or p55γ) and one 110-kDa catalytic subunit (p110α, β or δ). The Class IB PI 3-kinase is also a dimer, composed of one regulatory subunit (p101 or p87) and one catalytic subunit (p110γ) (Wymann et al. 2003). Class I enzymes will utilize PI, PI[4]P, or PI[4,5]P2 as substrates in vitro, but are thought to primarily produce PI[3,4,5]P3 in cells.The crystal structure of the Class IB PI 3-kinase catalytic subunit p110γ was solved in 1999 (Walker et al. 1999), and crystal or NMR structures of the Class IA p110α catalytic subunit and all of the individual domains of the Class IA p85α regulatory subunit have been solved (Booker et al. 1992; Günther et al. 1996; Hoedemaeker et al. 1999; Huang et al. 2007; Koyama et al. 1993; Miled et al. 2007; Musacchio et al. 1996; Nolte et al. 1996; Siegal et al. 1998). However, a structure of an intact PI 3-kinase enzyme has remained elusive. In spite of this, studies over the past 10 years have lead to important insights into how the enzyme is regulated under physiological conditions. This chapter will specifically discuss the regulation of Class IA PI 3-kinase enzymatic activity, focusing on regulatory interactions between the p85 and p110 subunits and the modulation of these interactions by physiological activators and oncogenic mutations. The complex web of signaling downstream from Class IA PI 3-kinases will be discussed in other chapters in this volume.

Fig. 1. Domain structure of p85α and p110α

The p85α regulatory subunit contains Src-homology 3 (SH3), BCR-homology (BCR), Proline rich (PRD), and Src homology 2 (SH2) and domains. The inter-SH2 domain (iSH2) is an antiparallel coiled-coil that links the two SH2 domains. The p110α catalytic subunit contains the adapter binding domain (ABD), which binds to the iSH2 domain of p85, a Ras-binding domain (RBD), a C2 domain, a helical domain (formally called the PIK homology domain), and a kinase domain

Fig. 2

Space-filing views of the nSH2-iSH2-p110α[H1047R] crystal structure (Huang et al. 2007; Mandelker et al. 2009), facing the iSH2 domain from the side (a) or down its barrel with the nSH2 domain removed (b). (c, d) Schematic models of the same orientations. In (d), the nSH2 domain is shown only in outline, to allow the rest of the iSH2 domain and p110α to be seen. The sites of the oncogenic mutations in p85α (D560, N564 and Q572) and p110α (E542/E545/Q546 and H1047) are shown, as is the phosphopeptide binding site of the nSH2 domain

Fig. 3. Models of physiological and oncogenic activation of p85/p110 dimers

The models show a schematic of p110α bound to the nSH2-iSH2 fragment of p85. In normal cells, phosphoprotein binding to the nSH2 domain disrupts the inhibitory interface with the helical domain of p110α. In transformed cells, mutations of residues in the acidic patch in the helical domain (for example E545K) constitutively disrupt nSH2-helical domain interactions, leading to the deregulation of the enzyme