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Pregnancy Timeline by SemestersFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresFemale Reproductive SystemEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterSecond TrimesterFirst TrimesterFertilizationDevelopmental Timeline
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Home | Pregnancy Timeline | News Alerts | News Archive July 17, 2013

 

mRNA

Above: Ribosomes manufacture proteins based on mRNA instructions.
Each ribosome reads mRNA, recruits tRNA molecules to fetch amino acids,
and assembles the amino acids in the proper order.

A single mRNA message can be used over and over again to create thousands of identical proteins.


Created by the National Institute of General Medical Sciences
http://publications.nigms.nih.gov/insidethecell/chapter2.html






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Visualizing mRNA Transport in a Test Tube

Much of biomedical science – both mystifying and awe-inspiring to the lay public – depends on an unwavering focus on things that can’t be easily seen, like the inner-workings of cells, in order to determine how and why disease develops. New research provides a rare “picture” of the activity taking place at the single molecule level.

by Jennifer Nachbur

New research authored by Thomas Sladewski, a University of Vermont graduate student working in the laboratory of Kathleen Trybus, Ph.D., and colleagues, provides a rare “picture” of the activity taking place at the single molecular level — visual evidence of the mechanisms involved when a cell transports mRNA (or messenger RNA) to where a protein is needed to perform a cellular function.

The study appears in the June 30, 2013 Nature Structural and Molecular Biology.


The process of mRNA localization is critical to cell function.

When defects in mRNA transport take place, human diseases, such as spinal muscular atrophy and Alzheimer’s disease, can occur. The transport of mRNAs is also important for neural development and synapse plasticity — needed for learning and memory.


According to Trybus, a UVM professor of molecular physiology and biophysics, ensuring proper cellular function is challenging. “The proteins responsible for orchestrating this task are not uniformly distributed, but often need to be in a certain place at a certain time,” she says.


mRNA plays a role in helping proteins reach target cells by using a unique identifier signal in the mRNA called a “zip code” to ensure mRNA identifies the place where the protein is needed.


“Just like the address on a piece of mail, these zip codes help transport the mRNA by linking up with a tiny molecular motor called myosin, which walks on a track called actin, carrying the mRNA to its destination,” Trybus explains.

Sladewski says that most mRNAs that are transported actually have multiple zip codes, which is like writing the address four times on a piece of mail.


“In our study, one question that we asked is why mRNAs have so many zip codes when it only needs one to be transported?” he says.

This redundancy, it turns out, is very important. “At the cell level, recruitment of a myosin motor to a zip code increases probability of engagement with the intended destination,” Sladewski explains.

A mRNA with one zip code has a probability of engaging with either zero or one myosin motor, but a mRNA with four zip codes will engage with between zero to four myosin motors. In this case, the multiple zip codes – redundancy – ensure that the mRNA will engage with at least one motor and will be successfully transported to its destination.


“You can think of this like shipping a box,” says Sladewski. “We normally address only one side of a box so the carrier needs to search up to six sides of the box to find the address, but, if you put the address on each of the six sides, no matter which orientation the box is in, the mailman knows its destination without having to search for it.”

To create the visual of these single molecules – mRNPs – moving in real time, Trybus, Sladewski and colleagues reconstructed all of the components essential to transporting mRNA in the cell and then used an imaging technology called total internal reflection microscopy (TIRF).

“mRNPs are really tiny – approximately 1000 times smaller than the width of a human hair – which makes them challenging to see,” Sladewski says. In order to see these mRNPs moving on even smaller actin tracks, as well as distinguish the mRNP from the actin, he and the research team attach a miniscule red probe on the actin and a green probe on the mRNP. “When we mix them and image with a sensitive TIRF microscope, we see green mRNPs moving on red actin tracks with high spatial resolution in real time,” he adds.

“By visualizing single mRNAs being carried to their destination in a test tube, we were able to understand how mRNA is moved in the cell, including how features of the myosin motor, as well as the cargo mRNA being transported, both influence the transport properties of the motor/mRNA complex,” Trybus says.

Trybus, Sladewski and colleagues’ study has provided a new tool for studying mRNA localization at a molecular level, which will help advance this important field of research.

Abstract
Molecular motors are instrumental in mRNA localization, which provides spatial and temporal control of protein expression and function. To obtain mechanistic insight into how a class V myosin transports mRNA, we performed single-molecule in vitro assays on messenger ribonucleoprotein (mRNP) complexes reconstituted from purified proteins and a localizing mRNA found in budding yeast. mRNA is required to form a stable, processive transport complex on actin—an elegant mechanism to ensure that only cargo-bound motors are motile. Increasing the number of localizing elements ('zip codes') on the mRNA, or configuring the track to resemble actin cables, enhanced run length and event frequency. In multi–zip-code mRNPs, motor separation distance varied during a run, thus showing the dynamic nature of the transport complex. Building the complexity of single-molecule in vitro assays is necessary to understand how these complexes function within cells.

Original press release:http://www.uvm.edu/medicine/?Page=news&storyID=16398&category=comall