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    • br PIK superfamily model of

    2020-07-29


    PIK superfamily: model of PI3Kα (p110α/p85α complex) Phosphoinositide lipid kinases (PIKs) are enzymes that catalyze the generation of specific phosphorylated variants of phosphatidylinositols (PtdIns) (Fruman et al., 1998). This superfamily of lipid kinase can be further divided into three major families according to the position of the inositol ring that get phosphorylated: the PtIns 3-kinases (PI3Ks), PtdIns 4-kinases (PI4Ks), and PtdIns-P (PtdinP) kinases (PIP4K and PIP5K) (Brown and Auger, 2011). Among all the PIKs, the structure of PI3K have been studied intensively likely because it\'s central role in tumor biology (Braccini et al., 2015; Costa et al., 2018; Follo et al., 2015; Janku et al., 2018; Liu et al., 2018). PI3Ks are a family of lipid kinases capable of catalyzing the phosphorylation of the 3′-hydroxyl group of the inositol ring. Based on primary structure and regulatory domain similarity, PI3Ks are further grouped into three corticosterone synthesis (Leevers et al., 1999). Here we just focus on PI3Kα, one member from class I, as an example to explore how the molecular structure of PIK is correlated to its specific catalytic mechanism. Unlike DGKB and SK1, which are homodimer and monomer in solution, respectively, PI3Kα is a heterodimer composed of an 85 kD regulatory subunit (p85α) and a 110 kD catalytic subunit (p110α). Each subunit contains 5 domains respectively as illustrated in Fig. 2A. p110α contains an adaptor-binding domain (ABD), a Ras-binding domain (RBD), a C2 domain, a helical domain, and a kinase domain (Amzel et al., 2008; Gabelli et al., 2010; Huang et al., 2007). p85 is composed of an SH3 domain, a GAP domain, an N-terminal SH2 (nSH2) domain, an inter-SH2 domain (iSH2), and a C-terminal SH2 domain (cSH2) (Gabelli et al., 2010; Huang et al., 2007). As the full-length structure of p110α/p85α complex is not available, a truncated enzymatic active complex p110α/niSH2, which contains full length of p110α and the nSH2/iSH2 domain of p85α was used to illustrate the organization of PI3Kα (Huang et al., 2007). The overall shape of p110α/iSH2 is dramatically different from DGKB or SK1, instead of forming a homodimer, p110α/iSH2 heterodimer has a triangular shape with iSH2 laying on the top and ABD, RBD, helical and kinase domain of p110α sitting in the bottom (Fig. 2B) (Gabelli et al., 2010; Huang et al., 2007). The C2 domain of p110α is in close proximity to iSH2 and both of them are thought to directly contact the membrane (Gabelli et al., 2010; Huang et al., 2007). While the N terminus of p110α is mainly involved in interacting with the regulatory subunit, the C-terminal catalytic domain of p110α maintains a conserved two lobes structures of αβ fold common to general kinase. The ATP binding site locates in the cleft of the two lobes as illustrated in Fig. 2B (Gabelli et al., 2010). With respect to its catalytic mechanism, PI3Ks have an interesting history. While this enzyme is known for its lipid kinase activity, it also has protein kinase activity (Dhand et al., 1994; Foukas and Shepherd, 2004). This has led to the recognition that it is has dual kinase specificity: a lipid kinase activity that catalyzes the ATP-dependent phosphorylation of the 3′-hydroxyl of phosphoinositides and a protein-kinase activity that includes the catalysis of autophosphorylation (Dhand et al., 1994; Foukas and Shepherd, 2004). Recently, critical catalytic residues have been identified as well as and a potential catalytic mechanism involving a ternary complex with ATP and PI(4,5)P2 has been proposed (Maheshwari et al., 2017) The data indicate that Lys776, within the P-loop region, is involved in both lipid and ATP substrate recognition and is critical for autophosphorylation. One interesting observation involved the roles of His936 and His917. While mutation of His917 abolished both lipid and autophosphorylation, mutation of His936 abolished lipid kinase activity only. This is similar to studies of another lipid kinase, a PtdIns-5 kinase (PIKfyve), that also displays protein kinases activity. A double mutation in a putative lipid substrate binding region of this enzyme (K1999E/K2000E) ablates lipid kinase activity but not protein kinase activity (Ikonomov et al., 2002).