Tubulin is a globular protein of which there are two forms; alpha and beta. These "homologous 50 kDa subunits. . . assemble in a helical array of alternating tubulin types" to form microtubules. Electron crystallography was used to analyse the 3-D strucutre of tubulin, it was found that "tubulins are members of the P-loop NTPase family and contain a nucelotide-binding site adjacent to the P-loop". It was also deteremined that alpha and beta types show high levels of similarity. It is thought that tubulin only exists in eukaryotes, although a prokaryote homolog has been identified, protein FtsZ. "This protein particitpates in bacterial cell division, forming ring structures. These observations suggest that tubulins may have been evolved from ancient cell-division protein."