Perturbation of the phosphatidyl­inositol 3­kinase (PI3K) pathway has long been associated with cancer development, with recent results identifying mutations that commonly occur in PIK3CA, the gene encoding the p110α catalytic subunit. Bert Vogelstein, Sandra Gabelli, Mario Amzel and colleagues have determined the structure of the complex formed between p110α and part of its regulatory subunit (p85α, encoded by PIK3R1) in an attempt to further understand the nature of the common mutations in PIK3CA.

Using a baculovirus system the authors were able to produce stable levels of p110α only when coexpressed with p85α. However, this led to protein aggregation, so the authors determined whether different truncated versions of p85α would improve solubility. Expression of residues 322–600 of p85α, which contain the inter­SH2 domain (iSH2) that binds p110α and the amino­ter­minal SH2 domain (nSH2), gave the highest protein yield. The complex was catalytically active and the result­ing p110α–niSH2 crystals were used to determine the structure to 3.05 Å resolution.

Mutations in PIK3CA have been found in the N­terminal adaptor binding domain (ABD), the C2 domain thought to interact with membranes, the helical domain of unknown function and the kinase domain. By examining the crystal structure the authors were able to more fully understand how these

mutations disrupt the function of p110α. For example, Arg 38 and Arg 88 in the ABD are often mutated to cysteine, histidine or glutamine and the new structure indicates that these mutations alter the interaction of the ABD with the kinase domain, changing the activity of the enzyme. Mutations in the C2 domain were thought to alter the interaction of p110α with the cell membrane. However, the crystal structure data indicate that mutation of Asn 345 in the C2 domain disrupts the interaction with the iSH2 domain of p85α, leading to p110α activation. Moreover, although less common, a mutation in p85α that leads to truncation of the protein at residue 571 was thought to constrain the function of the p85α inhibitory domain. However, results from the structure indicate that the truncation destabilizes the coiled­coil part of iSH2, disrupting the interaction with Asn 345 in p110α.

The new structure was also able to confirm some previous biochemi­cal findings. The helical domain of p110α has two mutational hotspots at Glu 542 and Glu 545, and to a lesser degree at Gln 546, which are located on an exposed region. Biochemical analyses had indicated that these residues interact with the N­terminal SH2 domain of p85α, inhibiting p110α. As this part of the p85α subunit was not well­resolved in the crystal structure, the authors employed modelling techniques using previous crystal structures.

Their findings indicate that mutation of these residues does prevent the inhibitory effect of the N­terminal p85α domain.

Hopefully, the insights provided by this new crystal structure will prove useful for the design of p110α­specific or mutation­specific drugs.

Nicola McCarthy

Original research PaPer Huang, C.-H. et al. The structure of a human p110α/p85α complex elucidates the effects of oncogenic PI3Kα mutations. Science 318, 1744–1748 (2007)

s i g n a l l i n g

Structural survey

Image courtesy of Nicola McCarthy

R e s e a R c h h i g h l i g h t s

NATUrE rEVIEwS | cancer VolUME 8 | FEBrUAry 2008

Nature Reviews Cancer | AoP, published online 18 January 2008; doi:10.1038/nrc2315

© 2008 Nature Publishing Group