Microtubules are hollow tubes some 25 nm in diameter participating in the 
eukaryotic cytoskeleton. They are built from alphabeta-tubulin heterodimers that 
associate to form protofilaments running lengthwise along the microtubule wall 
with the beta-tubulin subunit facing the microtubule plus end conferring a 
structural polarity. The alpha- and beta-tubulins are highly conserved. A third 
member of the tubulin family, gamma-tubulin, plays a role in microtubule 
nucleation and assembly. Other members of the tubulin family appear to be 
involved in microtubule nucleation. Microtubule assembly is accompanied by 
hydrolysis of GTP associated with beta-tubulin so that microtubules consist 
principally of 'GDP-tubulin' stabilized at the plus end by a short 'cap'. An 
important property of microtubules is dynamic instability characterized by 
growth randomly interrupted by pauses and shrinkage. Many proteins interact with 
microtubules within the cell and are involved in essential functions such as 
microtubule growth, stabilization, destabilization, and interactions with 
chromosomes during cell division. The motor proteins kinesin and dynein use 
microtubules as pathways for transport and are also involved in cell division. 
Crystallography and electron microscopy are providing a structural basis for 
understanding the interactions of microtubules with antimitotic drugs, with 
motor proteins and with plus end tracking proteins.