Scientists create new fluorescent protein to monitor the cell cycle

By Ryan Whitwam

Cellular mitosis is the basis of all higher life. When you need new cells, they can be created through a process called mitosis, whereby a cell makes a copy of itself. This process is highly regulated within the body, but sometimes those mechanisms fail and cells can grow out of control. We call that cancer. Medical researchers have an interest in knowing what phase of the cell cycle a specimen is going through, but it’s been hard to do that with live cells. A new technique from Stanford (PDF) allows the cell cycle to be observed with more precision than ever before.

There are four phases in the standard cell cycle: rest (G1), synthesis (S), rest (G2), and mitosis (M). The most popular way to visualize live cells doing their thing was with a fluorescent dye called Fucci (fluorescence ubiquitination cell cycle indicator). However, the problem with this method is that Fucci can’t tell you which phase a cell is in, only whether or not it’s divided. Fucci uses a red fluorescent protein to detect a protein called cdt1. It’s only present in the G1 phase. The green marker detects a protein called geminin, which is present in the S, G2, and M phases. So, these two fluorescent markers can tell you only if a cell is dividing or not.

The innovation in the Stanford research allows for a system where each of the phases is distinguishable. The team ran a series of 26 mutations on a red fluorescent protein marker in order to create a new color that can be useful in visualizing the cell cycle. They call the new marker mMaroon1, and the system it’s a part of is called Fucci4 (because it uses four different fluorescent markers).

When using Fucci4 to monitor a cell, a tweaked version of the a blue marker is used to light up cells that are in either G1 or S phases. A yellow marker that only light sup in the G1 phase differentiates that one from S, which is also when the green marker we talked about earlier lights up. That identifies the S phase. When you see only green, that’s G2. The M phase is where mMaroon1 comes into play. It lights up only during mitosis, and then shuts off when the cell enters G1 again. You can tell that happens when the yellow signal returns. So, all four phases are covered.

This tool could allow researchers to more easily monitor the effects different substances have on living cells. When the cells can continue living rather than being harvested and stained, the cycle can continue as we watch and gather data.