DO PROKARYOTES CONTAIN MICROTUBULES

In eukaryotic cells, microtubules are 24-nm-diameter tubular structure s composed of a class of conserved proteins called tubulin. They are i nvolved in numerous cell functions including ciliary motility, nerve c ell elongation, pigment migration, centrosome formation and chromosome movement. Although cytoplasmic tubules and fibers have been observed in bacteria, some with diameters similar to those of eukaryotes, no ho mologies to eukaryotic microtubules have been established. Certain gro ups of bacteria including azotobacters, cyanobacteria, enteric bacteri a, and spirochetes have been frequently observed to possess microtubul e-like structures, and others, including archaebacteria, have been sho wn to be sensitive to drugs that inhibit the polymerization of microtu bules. Although little biochemical or molecular biological information is available, the differences observed among these prokaryotic struct ures suggest that their composition generally differs among themselves as well as from that of eukaryotes. We review the distribution of cyt oplasmic tubules in prokaryotes, even though, in all cases, their func tions remain unknown. At feast some tend to occur in cells that are la rge, elongate, and motile, suggesting that they may be involved in cyt oskeletal functions, intracellular motility, or transport activities c omparable to those performed by eukaryotic microtubules. In Escherichi a coli, the FtsZ protein is associated with the formation of a ring in the division zone between the newly forming offspring cells. Like tub ulin, FtsZ is a GTPase and shares with