1. What is a molecular motor?
It's any protein that uses chemical energy, specifically ATP hydrolysis, to produce physical force.
2. How many types of motors are there?
Proteins that transport molecules and vesicles along the cytoskeleton; enzymes involved in DNA strand separation and replication, such as helicases, gyrases, and topoisomerases; and ATPases that move ions and large organic molecules across membrances are all motor proteins. Their genes have the same sequences to encode their ATP-hydrolyzing motor domains.
3. Which motors are intracellular transporters?
There are three superfamilies, comprising myosins, which travel along actin filaments, and kinesins and dyneins, which travel along microtubules. ATP hydrolysis causes a conformational change in these proteins' globular motor domains, allowing them to "walk" along their associated filaments (see Feature, p. 19). These superfamilies contain dozens of individual proteins, and new ones are still being characterized.1
4. What do they do?...
5. What happens if these proteins are mutated or deleted?
While some motor protein mutations are lethal, killing an embryo during development, others cause nonlethal defects. In cardiac muscles, missing myosins cause adult-onset cardiac diseases, such as hypertrophic cardiac myopathy. Mutations in myosin VI, which controls stereocilia movement in ear hair cells, lead to deafness in mammals. Differences in coat color occur when myosin V mutates, since it transports melanocyte vesicles. Changes in dyneins, responsible for retrograde axonal transport, also might be involved in neurodegenerative diseases such as amyotrophic lateral sclerosis.
- Maria W. Anderson