Bacterial tubulin BtubA/B: a folding mystery

Abstract

αβ-Tubulin heterodimers, from which the microtubules of the cytoskeleton are built, have a complex chaperone-dependent folding pathway. They are thought to be unique to eukaryotes, whereas the homologue FtsZ can be found in bacteria. The exceptions are BtubA and BtubB from Prosthecobacter, which have higher sequence homology to eukaryotic tubulin than to FtsZ. Some of their properties are different from tubulin, such as weak dimerization and chaperone-independent folding. However, their structure is strikingly similar to tubulin including surface loops, and BtubA/B form tubulin-like protofilaments. Also, the protein packing of the crystallized heterodimer resembles protofilaments due to the P6522 space group (see figure). Presumably, BtubA/Bwere transferred froma eukaryotic cell by horizontal gene transfer because their high degree of similarity to eukaryotic genes is unique within the Prosthecobacter genome. The results indicate that eukaryotic tubulin’s dependence on chaperones lies in the amino acid sequence and not in the overall fold. The chaperones might have a regulatory function [1].