A protein that acts as a motor (it moves stuff). Motor proteins are essential to life as we know it.
Myosins are the responsible for movement along the
thin filaments of
actin in our cells. Specifically, The two most common kinds of
Myosin are
Myosin I, which moves filaments themselves and vacuoles full of things like
neurotransmitters that are essential to
thought, and
Myosin II, which is important for the motion of muscles (see
sarcomere).
Dyneins and
Kinesins are motor proteins that move on
microtubules and are essential to
mitosis (cell reproduction),
cilia, and
flagella like the tail of sperm cells. These are also important for
transport and cell structure.
While there are motor proteins for
microfilaments and
microtubules, there are no known motor proteins for
intermediate filaments probably because intermediate filaments are non-polar, so the motor would have a very hard time moving in one direction consistantly.