PI3Ka, one of the class IA PI3Ks, is highly mutated in cancers. All mutations analyzed result in an
increase in enzymatic activity. The structures of this enzyme determined by X-ray diffraction, provide
a framework for analyzing the possible structural effect of these mutations and their effect on the
enzymatic activity. Many of the mutations occur at domain interfaces where they can affect domain
interactions and relieve the inhibition of the wild-type enzyme by the nSH2 domain of p85. This
mechanism is analogous to the mechanism of physiological activation by activated tyrosine-kinase
receptors in which the phosphorylated tyrosine of the receptor (or their substrates) dislodges the nSH2
from its inhibitory position in the complex by competing with its binding to a loop in the helical
domain. Other mutations in the kinase domain can directly affect the conformation of the catalytic site.
One mutation, His1047Arg, uses a completely different mechanism: it changes the conformation of the
C-terminal loop in such a way that it increases the interaction of the enzyme with the membrane,
granting increased access to the phosphoinositide substrates.