Figure 9. Schematic representation of putative molecular mechanisms induced by mAb(A)_p110γ resulting in discriminative inhibition of the PI3Kγ variants.

(A) Effect of mAb(A)_p110γ on the basal states of the PI3Kγ variants. Binding of mAb(A)_p110γ to the C2 domain mediates allosteric modulation of residues 551–650 in the helical domain and residues 1035–1050 located in the helices kα9 and kα10 of the C-terminal lobe of the kinase domain. These helices play important role in allosteric activation of p110γ, as it was shown in the case of Ras stimulation [35]. mAb(A)_p110γ -induced structural change of the kinase domain may interfere and reduce the basal lipid kinase activities of p87-p110γ and p101-p110γ. Slight protection of p101-p110γ basal lipid kinase activity from the inhibitory effect of mAb(A)_p110γ is in line with the previous data showing stimulatory modulation of p110γ by p101 independently of its Gβγ adaptor function [41]. (B) Effect of mAb(A)_p110γ on the PI3Kγ variants stimulated by Gβ₁γ₂. Binding of mAb(A)_p110γ to the C2 domain of p110γ causes allosteric exposure of a region (residues 551–650) in the helical domain which also includes crucial amino acids involved in interaction with Gβγ, Arg₅₅₂ and Lys₅₅₃ [48]. This results in allosteric interference of mAb(A)_p110γ with Gβγ binding to p110γ. p101 was shown to be also involved in interaction with Gβγ via putative Gβγ binding domain (GβγBD) located in the C-terminal region of p101 [48]. In contrast to p101, p87 contributed much lesser (if at all) to Gβγ interaction [28,38,41,48]. In the scenario of discriminative inhibition, mAb(A)_p110γ disrupts p110γ-Gβγ interaction in a similar way for each PI3Kγ variant, whereas unaltered Gβγ binding capacity of p101 still allows effective translocation of p101-p110γ and regulatory activity. In contrast, p87-p110γ showed reduced capability to interact with Gβγ in the presence of mAb(A)_p110γ resulting in drastic reduction of enzymatic activity.

Indicated are phosphatidylinositol 4,5-bisphosphate (PIP₂), phosphatidylinositol 3,4,5-trisphosphate (PIP₃), the Ras binding domain (RBD, residues 220–311), the C2 domain (residues 357–522), the helical domain (residues 545–725), the kinase domain (residues 726–1092) of p110γ [58], and putative Gβγ binding domain (GβγBD) of p101 [48].