Thick (24 nanometers in diameter) tubes made up of alpha-beta
tubulin dimers that are in almost every kind of
cell you can think of.
These filaments are polar. They have a + end which is more dynamic(grows and shrinks faster) than the - end. At the end of the + end there is usually an especially dynamic region made up of
tubulin that has its alpha and beta subunits bound to a nucleotide-triphosphate (like
ATP, but unlike
microfilaments, it is usually bound to
GTP). The middle and - end are, for the most part tubulin with a bound nucleotide-diphosphate (like
ADP, but usually is actually
GDP) on the beta subunit, and a nucleotide-triphosphate bound to the alpha subunit.
Along with the motor proteins that function on them (like
dyneins and
kinesins, microtubules are responsible for much of cell motility. Unlike the
pseudopod, cell extension and crawling seen as a result of
microfilaments,
microtubules are seen in more dramatic situations like the separation of
chromosomes in
mitosis, and the
flagella (tails) of
sperm and other cells, and even the
cilia (little hair-like projections which have highly coordinated beating paterns to move the cell and sometimes help it eat.
It may also be helpful to read about
microtubule associated proteins (MAPs).
See also
intermediate filament, and
microfilament.