Cellular folate binding proteins; function and significance.

It appears that specific functions may be assigned to some of the cellular folate binding proteins with some degree of certainty. Those that are membrane bound or derived from membranes probably have a role in transport of folate molecules into the cell. This is in spite of the fact that the localization of this protein to the plasma membrane has been carried out in only a limited number of cases. The role of the folate binding protein of L. casei in transport is much clearer. Bacteria provide the opportunity to obtain mutants defective in both transport and binding, and such mutants are more difficult to obtain with mammalian cell lines. The intracellular folate binding proteins have been discovered so far only in liver. The fact that the folate binding proteins in rat liver mitochondria are two enzymes, dimethylglycine dehydrogenase and sarcosine dehydrogenase, suggests that enzyme activities may eventually be discovered for the other intracellular folate binding proteins. This may not possible, however, and a reasonably strong case has been made that the folate binding protein of cytosol, FBP-CII, serves in a storage role. Such a storage role is difficult to prove since it depends, in part, on the demonstration that the protein becomes progressively less saturated during deficiency--a situation true also for enzymes.