Two studies, both last week in the Proceedings of the National Academy of Sciences, reported new molecular details on how cells get proteins to where they're going.
The first paper, available in the early online edition of PNAS, reported on how neurons transport proteins to their terminals after they are synthesized in the cell body. The transport problem is especially pressing in neurons because the proteins sometimes must travel extremely long distances: The longest axon in a human body, running from the base of the spine to the toes, can be more than 3 feet long.
The researchers reported that polystyrene beads conjugated to a piece of amyloid precursor protein or APP and injected into the cell body make tracks for the synapse. APP plays a well-researched role in Alzheimer's disease, but its function in healthy cells is unclear as yet.
Bearer's team attached the C-terminus of APP to polystyrene beads that are roughly the size of a viral particle. The N-terminus of APP was used as a negative control.
The researchers found that beads conjugated to the C-terminal of APP literally departed in a cloud of dust: "Beads conjugated to APP-C began transport immediately upon injection and already formed a plume heading toward the synapse by the time (one minute to three minutes) the axon was transferred to the confocal microscope," the authors wrote. Beads conjugated to the N-terminal of, in contrast, weren't going anywhere: "As expected, APP-N beads remained stationary at the injection site for one hour of observation."
"Until now, we didn't know what actually attaches the cargo to the motor and gives it a ZIP code' or address to ship it to. Our work shows that the cargo-motor hitch is as simple as a short peptide," said senior author Elaine Bearer, professor of pathology and laboratory medicine (and, in a pretty unusual combination, composer in the music department) at Brown University .
The C terminus of APP is highly conserved, spanning the evolutionary tree from fruit flies to humans. (Bearer and her colleagues used giant squid axons in their studies.) That suggests the peptide plays a universal role in cargo trafficking inside neurons and other cells.
Bearer said the research results pointed to APP transport as a possible drug target for Alzheimer's, Huntington's and other diseases characterized by a breakdown in nerve cell transport. The peptide tag could be used to tag protein therapy aimed at repairing synapses damaged by disease or by poisons such as lead.
It also might be used in both basic research and diagnostic studies, to determine whether and how much transport has been affected by dementia or to trace normal or abnormal neuronal circuits.
The paper was published by researchers from Brown University in Providence, R.I.; the Marine Biological Laboratory in Woods Hole, Mass.; and the National Institute of Neurological Disorders and Stroke, in Bethesda, Md.
In its Oct. 24, 2006, print issue, PNAS also reported more basic findings on intracellular transport. Researchers from the Max Planck Institute for Molecular Cell Biology and Genetics in Dresden, Germany, and the University of Florida reported molecular details of how kinesin-1, the cellular transport motor, interacts with the microtubulular highway.
Kinesin-1 consists of a head section that attaches to the microtubules, a tail section that has the protein that needs to be transported attached, and a midsection that - well, no one really knows what the midsection does. Senior author Stefan Diez and his colleagues suspected that the midsection might serve to prevent cellular traffic jams by holding the protein cargo a certain distance from the microtubules. They reasoned that the cell might provide a gap that would prevent the kinesin and its cargo from getting stuck on microtubule-associated proteins.
The researchers used a certain type of high-resolution microscopy known as fluorescence interference contrast, or FLIC microscopy, to measure the distance between the microtubule and cargo molecules. At only 17 nanometers, the cargo is surprisingly close to the rails, especially given that the kinesin molecule is 60 nanometers long when extended. However, the researchers argued that "a compact, but flexible, molecule may have biological advantages: 15 nm to 20 nm is likely to be long enough to allow the motor to pass MT-associated proteins on the MT surface, and the flexibility may allow a wide range of orientations of the cargo so that a single motor can move relatively unimpeded through a crowded cytoplasm and many motors can work together even when not aligned on the cargo surface."