Regulation of Class IA PI 3-kinases: C2 domain-iSH2 domain contacts inhibit p85/p110 alpha and are disrupted in oncogenic p85 mutants

We previously proposed a model of Class IA PI3K regulation in which p85 inhibition of p110 alpha requires (i) an inhibitory contact between the p85 nSH2 domain and the p110 alpha helical domain, and (ii) a contact between the p85 nSH2 and iSH2 domains that orients the nSH2 so as to inhibit p110 alpha. We proposed that oncogenic truncations of p85 fail to inhibit p110 due to a loss of the iSH2-nSH2 contact. However, we now find that within the context of a minimal regulatory fragment of p85 (the nSH2-iSH2 fragment, termed p85ni), the nSH2 domain rotates much more freely (tau(C) approximate to 12.7 ns) than it could if it were interacting rigidly with the iSH2 domain. These data are not compatible with our previous model. We therefore tested an alternative model in which oncogenic p85 truncations destabilize an interface between the p110 alpha C2 domain (residue N345) and the p85 iSH2 domain (residues D560 and N564). p85ni-D560K/N564K shows reduced inhibition of p110 alpha, similar to the truncated p85ni-572(STOP). Conversely, wild-type p85ni poorly inhibits p110 alpha N345K. Strikingly, the p110 alpha N345K mutant is inhibited to the same extent by the wild-type or truncated p85ni, suggesting that mutation of p110 alpha-N345 is not additive with the p85ni-572(STOP) mutation. Similarly, the D560K/N564K mutation is not additive with the p85ni-572(STOP) mutant for downstream signaling or cellular transformation. Thus, our data suggests that mutations at the C2-iSH2 domain contact and truncations of the iSH2 domain, which are found in human tumors, both act by disrupting the C2-iSH2 domain interface.