Homologs of α- and β-tubulin have been identified in the "Prosthecobacter" genus of bacteria. They are designated BtubA and BtubB to identify them as bacterial tubulins. Both exhibit homology to both α- and β-tubulin. While structurally highly similar to eukaryotic tubulins, they have several unique features, including chaperone-free folding and weak dimerization. Cryogenic electron microscopy showed that BtubA/B forms microtubules "in vivo", and suggested that these microtubules comprise only five protofilaments, in contrast to eukaryotic microtubules, which usually contain 13. Subsequent "in vitro" studies have shown that BtubA/B forms four-stranded 'mini-microtubules'.