Thin (7-9 nanometer) filaments of
actin 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
actin that is bound to a
nucleotide-triphosphate (like
ATP). The middle and - end are for the most part actin with a bound nucleotide-diphosphate (like
ADP). The + end can be determined by "decorating" a filament with
Myosin S1 as described in the write-up on
Myosin.
Along with the motor proteins that function on them, microfilaments are responsible for much of cell
motility.
See also
F-actin,
intermediate filament, and
microtubule.