58K was previously identified as a rat liver protein that binds microtubules in vitro and is associated with the cytoplasmic surface of the Golgi apparatus in vivo (Bloom, G. S., and Brashear, T. A. (1989) J. Biol. Chem. 264, 16083-16092). We now report that 58K is a formiminotransferase cyclodeaminase (FTCD), a bifunctional enzyme that catalyzes two consecutive steps in the modification of tetrahydrofolate to 5,10-methenyl tetrahydrofolate. Comparative immunoblotting using several monoclonal antibodies made against 58K and a polyclonal antibody made against 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-purification of protein recognized by all antibodies through multiple fractionation steps, including gel filtration and ion exchange chromatography, and sucrose gradient ultracentrifugation. Eight peptides derived from 58K showed high sequence identity to amino acid sequences predicted by full length cDNA for p60 and porcine liver FTCD. Furthermore, purified 58K was associated with formiminotransferase and cyclodeaminase activities. Based on these collective results, 58K was concluded to be a rat liver version of FTCD. Microtubules assembled from brain tubulin, but not from liver tubulin, were able to bind rat liver FTCD. Binding to brain microtubules is suspected to occur via polyglutamates that 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 physiological significance of the microtubule binding activity of FTCD is thus called into question, but an association of FTCD with the Golgi apparatus has now been established.