Divergent β‐tubulins from foraminifera: Implications for microtubule assembly

Abstract

Foraminifera utilize an unusual and very rapid microtubule assembly/disassembly system: a state transition between microtubules and helical filaments. The helical filament is an assembly polymorph that forms when the microtubule lattice uncoils into a ribbon of tubulin dimers connected only by lateral subunit/subunit bonds. An unusual β‐tubulin sequence, which may be implicated in this process, has previously been reported from the foraminiferan Reticulomyxa filosa. In order to determine the taxonomic distribution and possible significance of this tubulin isoform, we sequenced and analyzed β‐tubulin genes from a broad taxonomic range of foraminifera. These genes contain a large number of substitutions, conserved within the group, which may alter the biochemical properties of β‐tubulin, especially in regions involved in subunit/subunit binding between α‐ and β‐tubulin in the microtubule lattice. In particular, the M‐loop, which is intimately involved in lateral subunit binding, shows only 30% homology to the consensus sequence for eukaryotic β‐tubulins. Multiple regions identified to be involved in longitudinal contacts with α‐tubulin are also highly substituted. In contrast, the foraminiferal α‐tubulin is well conserved with its homologs in other organisms. We suggest that these alterations to the consensus eukaryotic β‐tubulin sequence may help to explain the unusual assembly properties of foraminiferal microtubules. This work was supported by NSF OPP0003639.