42] Membrane-Associated folate transport proteins

Abstract

This chapter focuses on membrane-associated folate transport proteins. Folate compounds serve as coenzymes for various enzymes that catalyze reactions of one-carbon metabolism. Eukaryotic cells and certain bacteria are unable to synthesize folates de novo and therefore have developed transport systems to capture these compounds from the environment and to mediate their uptake into cells. Transport systems for folate compounds also account for the primary route by which a number of antifolates enter cells. Folate transport proteins have been identified in the plasma membranes of various cells. Membrane-associated folate transport proteins can be solubilized with the detergent Triton X-100 and purified by procedures adapted to hydrophobic proteins. A unique step, particularly useful in the purification of folate transporters, is adsorption and elution from microgranular silica. Noncovalent and covalent labeling techniques have been employed to monitor folate transport proteins during purification. Solubilization can also be achieved with octylglucoside and deoxycholate, but 5- to l0-fold higher concentrations are required. Bound [ 3 H]folate provides a convenient means for following the folate transport protein during purification and also for quantitating the yield and degree of purification achieved in each step.