Featured Article - May 2015
Short description: Crystal structures provide a glimpse into how phosphoinositides stabilize nuclear hormones.
Phosphoinositides have non-membrane-associated regulatory roles in the nucleus. Yet the exact function of the nuclear pool of these phospholipids remains unclear. Fletterick, Ingraham and colleagues (PSI JCSG and PSI STEMCELL) have shed light on one such role.
Previous work had suggested that phosphoinositides can act as ligands for nuclear receptors, binding to the hydrophobic pocket. To obtain a detailed view of such lipid-protein interactions and explore their biological significance, the authors solved the crystal structures of the lipid-binding domain (LBD) of human nuclear receptor SF-1 (also known as NR5A1) bound to the phosphoinositides PI(3,4,5)P3 (PIP3) (PDB 4QJR) or PI(4,5)P2 (PIP2) (PDB 4QK4). For both complexes, a peptide from coactivator protein PGC1-α was used as a cocrystallization factor.
The choice of phospholipids was dictated by the authors' binding assays, which indicated that among various phosphoinositides, these two species bound to SF-1 LBD with the highest affinity. Consistently, analysis of the melting temperature of SF-1 LBD bound to different phospholipid ligands showed that those two phosphoinositides had the greatest stabilizing effects. PIP3 had the strongest affinity and effect on the stability of SF-1 LBD; it also enhanced the affinity of SF-1 for the coactivator PGC1-α to a greater extent than did PIP2.
Structure-associated analyses revealed that the acyl chains of these phosphoinositides are buried in the hydrophobic, hormone-binding pocket of SF-1, whereas their charged, hydrophilic head group bulges out to be fully exposed to solvent. Moreover, the head groups of PIP3 and PIP2 form an extensive network of interactions with SF-1 loop L2-3 (linking helices H2 and H3) at the entrance of the hormone-binding pocket, stabilizing the loop and preparing SF-1 for association with transcription coregulators. Intriguingly, mutations in two of the loop residues mediating stabilizing interactions had been linked to the loss of function of SF-1 and with pathologies including male infertility, premature ovarian failure and adrenal insufficiency. Thus, this knowledge could guide therapeutic targeting of SF-1 beyond its hydrophobic pocket.
The team's earlier work hinted that PIP3-associated SF-1 is more active than the PIP2-bound version. Yet the two related structures are nearly identical, which may be due to the stabilizing effect of the PGC1-α peptide. The authors propose that PIP3's head group and the stabilized loops of SF-1 together form a docking surface for other nuclear proteins, acting as a regulatory site. Future structures of full-length SF-1 in complex with various phosphoinositides may provide more definitive answers.
R. D. Blind et al. The signaling phospholipid PIP3 creates a new interaction surface on the nuclear receptor SF-1.
Proc Natl Acad Sci U S A. 111, 15054-9 (2014). doi:10.1073/pnas.1416740111