58K, A MICROTUBULE-BINDING GOLGI PROTEIN, IS A FORMIMINOTRANSFERASE CYCLODEAMINASE

58K was previously identified as a rat liver protein that binds microt ubules in vitro and is associated with the cytoplasmic surface of the Gels apparatus in vivo (Bloom, G. S., and Brashear, T. A. (1989) J. Bi ol. Chem. 264, 16083-16092). We now report that 58K is a formiminotran sferase cyclodeaminase (FTCD), a bifunctional enzyme that catalyzes tw o consecutive steps in the modification of tetrahydrofolate to 5,10-me thenyl tetrahydrofolate. Comparative immunoblotting using several mono clonal antibodies made against 58K and a polyclonal antibody made agai nst a chicken liver protein (p60) with similar properties (Hennig, D., Scales, S. J., Moreau, A., Murley, L. L., De Mey, J., and Kreis, T. E . (1998) J. Biol. Chem. 273, 19602-19611) demonstrated precise co puri fication of protein recognized by all antibodies through multiple frac tionation steps, including gel filtration and ion exchange chromatogra phy, and sucrose gradient ultracentrifugation. Eight peptides derived from 58K showed high sequence identity to amino acid sequences predict ed by full length cDNA for p60 and porcine liver FTCD. Furthermore, pu rified 58K was associated with formiminotransferase and cyclodeaminase activities. Based on these collective results, 58K was concluded to b e a rat liver version of FTCD. Microtubules assembled from brain tubul in, but not from liver tubulin, were able to bind rat liver FTCD. Bind ing to brain microtubules is suspected to occur via polyglutamates tha t are added post-translationally to tubulin in brain, which was shown to contain very low levels of FTCD, but not to tubulin in liver, which was determined to be the richest tissue source, by far, of FTCD. The physiologial significance of the microtubule binding activity of FTCD is thus called into question, but an association of FTCD with the Golg i apparatus has now been established.