Biobricks

KillerRed

KillerRed is a key protein in our bacterial density control system. It represents the light-sensitive element that allows the cells to receive signals from the control device.

Main Functions

KillerRed is a red fluorescent protein, meaning that by illuminating it with wavelengths from a certain portion of the visible spectrum, it re-emits light in another portion with longer (less energetic) wavelengths. Below is the absorption and emission spectra for the KillerRed protein:

The KillerRed protein absorption (left peak) and emission (right peak) spectra
Source:Detailed KillerRed description from Evrogen

From the emission and absorption spectra, we can determine that the protein absorbs in the green portion of the spectrum with a peak at 585 nm and emits in the red portion of the spectrum with a peak at 610 nm, hence the name "KillerRed".
Emitted light from bacteria is proportional to the amount of protein in the cells. This allows for measuring protein concentration in a cell culture.

The second function of the protein is emission of ROS (Reactive Oxygen Species) when fluorescing.
ROS are highly unstable and react chemically with many substrates including proteins, lipids and DNA. These reactions are oxidative and damage the affected molecules, making ROS toxic to the cell. With sufficient amounts of ROS, a cell's essential components can be damaged beyond repair, and the cell killed. Thus illuminating KillerRed-expressing cells with light in the green portion of the visible spectrum kills them, a mechanism that we use to control cell density in a culture.

Structure

In order to understand why KillerRed has its unique properties it is necessary to look at its structure. The protein is remarkably similar to other fluorescent proteins like GFP (Aequorea victoria> and dsRed (Discosoma striata), featuring a beta-barrel housing a central alpha helix with the fluorescent chromophore at its center. Normally the chromophore is protected from the outside environment by the protein shell, but this isn't the case with KillerRed.

A comparison of the 3D structures of monomerix dsRed (left) and dimeric KillerRed (right)
Credits to Carpentier P., Violot S., Blanchoin L., Bourgeois D. for the KillerRed structure, and Strongin D.E., Bevis B., Khuong N., Downing M.E., Strack R.L., Sundaram K., Glick B.S., Keenan R.J. for the dsRed structure. Source: RCSB protein database entries 2WIQ and 2VAD.