Identification and characterization of axopodial tubulins from Echinosphaerium nucleofilum

Isolated microtubule protein from axopodia of the heliozoan Echinosphaerium nucleofilum, consisting of two major bands on SDS-polyacrylamide gel electrophoresis (SDS-PAGE), has been compared to axonemal and cytoplasmic tubulins from both animal and non-animal sources. The upper E. nucleofilum protein band migrated faster than the alpha-tubulins of bovine brain and sea anemone sperm tails but with approximately the same electrophoretic mobility as the axonemal alpha-tubulins of Tetrahymena pyriformis and the alga Chlorogonium elongatum and cytoplasmic alpha-tubulin from the slime mold Physarum polycephalum. The lower E. nucleofilum protein band, however, had a higher electrophoretic mobility than all the beta-tubulins which we have so far examined. It was, nevertheless, a true beta-tubulin as shown by its migration on two-dimensional gel electrophoresis and the general resemblance of its one- and two-dimensional peptide maps to those of other beta-tubulins. The Staphylococcus aureus protease cleavage pattern of the upper axopodial protein band was similar to those of other non-animal alpha-tubulins but quite different from those of the animal alpha-tubulins. In contrast, the two-dimensional tryptic peptide map of axopodial alpha-tubulin was distinct from all of them. For example, a characteristic constellation of peptides common to the peptide maps of the other alpha-tubulins was absent from that of E. nucleofilum. In contrast to Physarum and metazoan tubulins but similar to Tetrahymena tubulin, the axopodial alpha-tubulin had a more basic isoelectric point than the beta-subunit as shown by two dimensional gel electrophoresis. Some of the unusual characteristics of E. nucleofilum axopodial tubulin may not only reflect phylogenetic variation, but also the different functional requirements of axopodial microtubules.