The folate and thiamine transport proteins of lactobacillus casei

✍ Scribed by Henderson, Gary B. ;Zevely, Edward M. ;Kadner, Robert J. ;Huennekens, F. M.

• Publisher: Wiley (John Wiley & Sons)
• Year: 1977
• Tongue: English
• Weight: 516 KB
• Volume: 6
• Category: Article
• ISSN: 0091-7419
• DOI: 10.1002/jss.400060209

✦ Synopsis

Abstract

Two separate binding proteins, one specific for folate and the other for thiamine, have been isolated from membrane fragments of Lactobacillus casei. Purification to homogeneity was achieved by fractionation of the Triton‐solubilized proteins with microgranular silica (Quso G‐32) and Sephadex G‐150. Amino acid analyses revealed that the folate (M~r~ = 25,000) and thiamine (M~r~ = 29,000) binders have unusually low polarity constants, 0.32 and 0.26, respectively. Evidence obtained with intact cells has established a direct role for these binding proteins in transport of the corresponding vitamins: (A) In each case, the processes of binding and transport showed similarities in substrate affinities and repression by excess vitamin in the growth medium. (B) Competition studies employing amethopterin, 5‐formyl tetrahydrofolate, and 5‐methyl tetrahydrofolate (for folate) and thiamine monophosphate and thiamine pyrophosphate (for thiamine) have shown that the ability of these compounds to inhibit the transport of the corresponding vitamins is paralleled by their ability to inhibit binding. (C) Amethopterin‐resistant mutants which are defective in folate transport have a comparable defect in ability to bind folate. (D) Amethopterin‐resistant cells which (compared with the parent cell line) contain folate transport systems with altered affinities for amethopterin also contain binding proteins whose affinities for amethopterin have changed by equivalent amounts. (E) Both the transport and binding of folate by one of the mutants were stimulated (approximately 3‐fold) in parallel by the addition of mercaptoethanol.