The p85a nSH2 domain interacts with three p110a domains, C2, helical and kinase (Fig. 1). A loop
of the helical domain containing residues 541 to 546 fits in a grove of the nSH2 domain providing
a strong interaction that holds the nSH2 in its inhibitory position (Mandelker et al., 2009). Aligning
the structure of the isolated nSH2 with bound peptide to the same domain in the p110/niSH2
structure shows that the peptide binds to the nSH2 domain in exactly the same position occupied by
residues 541-546 of the helical domain (Fig. 2). Thus, pY-pep binding competes with the major
interaction between the nSH2 domain and the p110a helical domain, dislodging the nSH2 domain
and thereby relieving the nSH2 inhibition resulting in activation of the enzyme. The loss of this
interaction is not sufficient to dissociate p85a from p110a due to the strong interactions of the ABD
and C2 with the iSH2 of p85.