c-Fos is a transcription factor rapidly and transiently transcribed upon stimulus in a cell. Two apparently unlinked functions have been described for this protein: it participates in normal and oncogenic cell growth through transcription regulation as an AP-1 dimeric complex with proteins of the Jun family. Independently of nucleus activity, c-Fos specifically modulates phospholipid metabolism. The structure of the c-Fos has not been resolved. The basic DNA-binding domain and the leucine zipper dimerization domain, but not the entire protein, has been crystallized associated to DNA. The c-Fos/c-Jun dimer AP-1 is stabilized by interfaces, like the heterodimeric protein importin along its way to the cell nucleus, and DNA to mediate transcription of target genes. As a monomer, c-Fos associates to endoplasmic reticulum ?that is, a membrane interface- to activate phospholipid metabolism. As a general rule, the interaction of c-Fos with biointerfaces lies at the core of its biological functions, and we first demonstrated that a clean air-water interface provides a strong thermodynamic stabilization of either pure c-Fos, c-Jun, or AP-1.
A variety of phospholipid (phosphatidylcholine, phosphaditdylserine, phosphatidylglycerol or phosphatidylinositols) interfaces further stabilize c-Fos.
Differential interactions with phospholipids induce changes of lipid organization affecting other membrane constituents or associated elements like the activity of enzymes mediating signal transduction events. These changes, involving at least two molecular reorderings of the protein, evidence its ability to sense membrane packing and composition, and result in modulation of phospholipase activity.
We used Small Angle X-Ray Scattering to explore the nature of the interactions of c-Fos with dimiristoylphosphatidylcholine (DMPC) vesicles, and with its AP-1 partner, c-Jun. A range of temperatures encompassing the DMPC gel-to-liquid chrystalline transition was covered. Although the results need better statistics, they suggest that below the lipid transition point, the bilayer thickness of DMPC vesicles is around 60 Å and does not change significantly by incorporation of c-Fos. We cannot exclude the possibility of some protein being adsorbed to the outer layer in the vesicles. Beyond the transition point the vesicle population apparently becomes polydisperse independently of the presence of the protein, perhaps with multilayer formation.
On the other hand, we were able to see AP-1 formation in solution when equimolar amounts of c-Fos and c-Jun are stablized by DNA that contains the AP-1 binding site. We conclude that SAXS is a suitable technique to study c-Fos/phospholipid and c-Fos/c-Jun interactions that complements monolayer studies.
