Dec 06 2018 01:00 PM
Dec 06 2018 03:00 AM
By Safia S. Aljedani
The phosphatidylinositol-3-kinase α (PI3Kα) is a heterodimeric enzyme that is composed of a
p85α regulatory subunit and a p110α catalytic subunit. PI3Kα plays a critical role in cell
survival, growth and differentiation, and is the most frequently mutated pathway in human
cancers. The PI3Kα pathway is also targeted by many viruses, such as the human
immunodeficiency virus (HIV-1), the herpes simplex virus 1 (HSV-1) or the influenza A virus,
to create favourable conditions for viral replication. The regulatory p85α stabilizes the catalytic
p110α, but keeps it in an inhibited state. Various ligands can bind to p85α and allosterically
activate p110α, but the mechanisms are still ill-defined. Intriguingly, p85α also binds to, and
activates, the PTEN phosphatase, which is the antagonist of p110α. Previous studies indicated
that only p85α monomers bind to the catalytic p110α subunit, whereas only p85α dimers bind
to PTEN. These findings suggest that the balance of p85α monomers and dimers regulates the
PI3Kα pathway, and that interrupting this equilibrium could lead to disease development.
However, the molecular mechanism for p85α dimerization is controversial, and it is unknown
why PTEN only binds to p85α dimers, whereas p110α only binds to p85α monomers. Here we
set out to elucidate these questions, and to gain further understanding of how p85α ligands
influence p85α dimerization and promote activation of p110α. We first established a
comprehensive library of p85α fragments and protocols for their production and purification.
By combining biophysical and structural methods such as small angle X-ray scattering, X-ray crystallography, nuclear magnetic resonance, microscale thermophoresis, and chemical crosslinking, we investigated the contributions of all p85α domains to dimerization and ligand binding. Contrarily to the prevailing thought in the field, we find that p85α dimerization and
ligand recognition involves multiple domains, including those that directly bind to and inhibit
p110α. This finding allows us to suggest a molecular mechanism that links p85α dimerization
and allosteric p110α activation through ligands.