Phosphoinositide 3-kinase δ is upregulated in lymphocytic leukemias. Because the 
p85-regulatory subunit binds to any class IA subunit, it was assumed there is a 
single universal p85-mediated regulatory mechanism; however, we find 
isozyme-specific inhibition by p85α. Using deuterium exchange mass spectrometry 
(DXMS), we mapped regulatory interactions of p110δ with p85α. Both nSH2 and cSH2 
domains of p85α contribute to full inhibition of p110δ, the nSH2 by contacting 
the helical domain and the cSH2 via the C terminus of p110δ. The cSH2 inhibits 
p110β and p110δ, but not p110α, implying that p110α is uniquely poised for 
oncogenic mutations. Binding RTK phosphopeptides disengages the SH2 domains, 
resulting in exposure of the catalytic subunit. We find that phosphopeptides 
greatly increase the affinity of the heterodimer for PIP2-containing membranes 
measured by FRET. DXMS identified regions decreasing exposure at membranes and 
also regions gaining exposure, indicating loosening of interactions within the 
heterodimer at membranes.