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Answer to the Friday Whatsit for May 7th

May 11, 2010

Friday whatsit

Click to enlarge

I see that no one tried to guess the Friday Whatsit last week. It was a tough one. The green things are in a sub-cellular structure (also known as an organelle) called the endoplasmic reticulum (ER for short).

The ER has two main functions: regulating levels of the messenger molecule calcium and constructing proteins for use inside the cell as well as for export to other cells. For the second function, the ER works in concert with another organelle, the Golgi apparatus. Side note: was The Golgi Apparatus also the name of a band? Readers in the know, please comment.

Anyway, let’s break this image down so you can see how it was made and discover what is making the ER in this cell green (hint: it’s not bad sushi).

First, this cell is not a typical cell. It has been forced to express a protein called green fluorescent protein (GFP). GFP is a naturally occurring bioluminescent (bio like biological, luminescent like light-making) protein made by the jellyfish Aequorea victoria. Biologists have figured out how to attach the gene for this protein to genes for other proteins normally found in cells, so the cells make proteins with GFP attached and we can see where they go in a living cell. The GFP proteins in this cell are attached to molecules of another protein that is normally found in the ER, which is the reason they hang around there after they are made instead of going all over the cell.

This picture is actually composed of two images of the same cell superimposed on each other.

cell only

The cell imaged with white light

ER in the cell

The GFP, made by collecting only green light

The first image is of the cell itself, made under the microscope with regular everyday white light (like room like or sunlight). The second image was made using only blue light, in this case a laser at a wavelength of 488nm, which sort of a teal color. In addition to being very fashionable, this teal light causes GFP to glow green, emitting light in the range of 510-560 nm. I captured this emission to make the second picture and superimposed it on the first image. Viola! The endoplasmic reticulum lights up green, and we can see where it is within the cell. This cell is (well, was at the time the picture was taken) alive, and if I had kept it happy instead of pouring bleach in the dish, I could have taken a sequence of images over several hours or days to see how the ER changes over time.

ER in lots of cells

A field of cells, some infected, some not

In case you were wondering: The ER protein gene with the GFP gene attached is on a small, circular bit of DNA called a plasmid. There are several ways to get a plasmid into a cell (a process called transfection) including packing it into small lipid (fat) bubbles and inducing the cells to take up those bubbles, or mixing the DNA and cells and then running electricity through the system to open pores in the cells and get the DNA to go in.

This transfection takes advantage of a virus. In the wild, viruses insert their own plasmids into cells and use the cell’s protein making machinery to replicate themselves. In a lab, the virus has been engineered to infect cells with only the DNA the scientist wants and not the DNA the cell would need to make more viruses. Bummer for the viruses, but great for the scientist because it is more efficient and easier on the cells than forcing them to take up lipid bubbles or zapping them with electricity. Still, not all cells will get the same amount of plasmid, and some cells will not be infected at all. Click on the image above to see a bunch of infected cells expressing different amounts of the ER-GFP protein.

Special thanks to University of Chicago community members Dr. Adam Hammond and his first year (class of 2009) BioPhysics Program graduate students for infecting these cells with the Invitrogen Organelle Lights ER-GFP kit and then leaving them behind for me to image.

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