Characterisation of the mechanism of translocation of FGF1 into cells

 
The translocation of FGF1 to the cytosol appears to occur from intracellular acidic vesicular compartments and requires the transmembrane potential. Brefeldin A which blocks retrograde transport through the Golgi apparatus to the endoplasmic reticulum did not prevent translocation. Introduction of artificial disulfide bonds in the growth factor did not prevent translocation to the cytosol. In the case of diphtheria toxin and ricin a disulfide in the A-chain blocks translocation, indicating that different translocation mechanisms are operating. In addition, the translocation of FGF1 to the nucleus is highly sensitive to inhibitors of PI 3-kinase, indicating that a 3-phosphoinositide regulates this process.

A major effort will be made to identify membrane proteins involved in the translocation. An attractive possibility is that the growth factor may be translocated by an ABC transporter present in the membrane of an intracellular organelle. Mammalian cells contain a large number of different ABC transporters which fulfil a number of different functions. We plan to test the possibility that such transporters participate in the translocation. One approach is to label ATP-binding proteins in the cell (among them the ABC transporters) to see if some of the labelled proteins bind to FGF1 or to its receptor. If so, we will try to identify the protein.