Cytoplasmic codiffusion of fatty acids is not specific for fatty acid binding protein

The intracellular movement of fatty acids is thought to be facilitated through codiffusion with fatty acid binding protein (FABP). Previous work suggested that FABP decreases fatty acid binding to immobile membranes, causing faster cytoplasmic diffusion. However, the specificity for binding to FABP has not been addressed. The aim of the current study was to determine whether specific FABP binding is required or whether binding to other proteins will produce the same effect. A model cytoplasm consisted of a fatty acid, proteins, and liposomes to simulate intracellular membranes. Laser photobleaching (fluorescence recovery after photobleaching) was used to measure the movement of the fluorescent fatty acid 12-N-methyl-7-nitrobenzo-2-oxa-1,3-diazoaminostearate (NBD-stearate) in model cytoplasm, in normal and permeabilized Hep G2 cells, and after incubation of permeabilized cells with bovine serum albumin (BSA) or FABP. Increasing protein in the model cytoplasm increased the diffusion rate in proportion to the extent of protein binding. Cell permeabilization reduced diffusion of NBD-stearate to < 5% of controls. Incubation of permeabilized cells with FABP or BSA resulted in a concentration-dependent increase in the NBD-stearate diffusion rate. BSA was more effective than FABP in binding NBD-stearate and increasing its diffusion rate after permeabilization. Proteins like FABP promote the diffusion of fatty acids. Removal of these proteins drastically reduces cytoplasmic diffusion. Substitution with BSA reestablishes the diffusive flux, suggesting that specific binding to FABP is not required. These data support a role for intracellular binding proteins in facilitating the cytoplasmic movement of fatty acids.