SH3 domains selectively activate the PI3 kinase through non-conventional tertiary contacts.

Phosphoinositide-3 kinase (PI3K) is a central regulator of cellular metabolism and survival, and its dysregulation is implicated in major human diseases, particularly cancer. The p85 regulatory subunit of PI3K uses its C-terminal domains to stabilise the catalytic p110 subunit in an inhibited state. Certain Src homology 3 (SH3) domains activate p110 by binding to the proline-rich (PR) 1 motif at the N-terminus of p85. How this interaction leads to PI3K activation remains unclear. Moreover, the low specificity of SH3 domains raises the question about how they can selectively control PI3K activation. Combining structural, biophysical, and functional methods, we demonstrate that both questions are linked: PI3K-activating SH3 domains form additional 'tertiary' interactions with the C-terminal domains of p85, relieving p110 inhibition. SH3 domains lacking these tertiary contacts may bind p85 with similar affinity but fail to activate PI3K. Thus, p85 employs a selection mechanism that discriminates based on binding mode rather than binding strength, preventing nonspecific activation rather than nonspecific binding. This mechanism conveys a functional selectivity to SH3 domains that are otherwise considered promiscuous. These insights establish a mechanistic framework that will help to predict, modulate, and therapeutically target SH3-driven PI3K activation in disease.