Purification and properties of a membrane-associated, folate-binding protein from Lactobacillus casei

A folate-binding protein has been solubilized from Lactobacillus casei by treatment of membrane preparations with Triton X-100 in the presence of [3H]folate. The protein-folate complex was purified 100-fold and recovered in a 22% yield by adsorption and elution from microgranular silica (Quso G-32), followed by passage through Sephadex G-150. When subjected to sodium dodecyl sulfate/polyacrylamide gel electrophoresis, the purified preparations showed only a single, protein-staining band whose molecular weight was 25,000. Bound folate (34 nmol/mg of protein) corresponded to 0.85 mol/mol of protein. Analyses of the protein revealed relatively few charged or polar amino acids, an unusually high content of hydrophobic residues and methionine, and the absence of cysteine. The purified protein-folate complex was contained within a Triton micelle (molecular weight, 220,000; about 340 mol of detergent per mol of protein). Bound folate was retained when the micelle was exposed at 4 degrees to solutions whose pH values ranged between 3 and 12; at 23 degrees, however, stability was decreased, especially above pH 8. Folate could be released by treatment of the micelle with ethanol or with chaotropic agents such as guanidinium chloride, perchlorate, or thiocyanate.