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Kerfeld lab reveals a new light-responsive protein family


Cyanobacteria, formerly known as blue-green algae, evolved on Earth billions of years ago. And these simple organisms – one cyanobacterial cell is 25 times smaller than a human hair – are extremely captivating to those who study them, including Dr. Matt Melnicki.

“Cyanobacteria were probably the first organisms to do photosynthesis with water,” says Matt, a postdoc in the Kerfeld Lab and one of the study’s primary researchers. “Almost every organism on the planet gets its energy ultimately from this kind of photosynthesis, and thus much of what is taught in biology classes depends on cyanobacteria evolving that capability.”

These underappreciated organisms, which dominate the oceans, are also the very reason we have a breathable atmosphere nowadays – after all, one of the byproducts of photosynthesis is oxygen, released into the air.

>Image of cyanobacteria under a microscope
Each cell 25 times smaller than a human hair.
By Eric Young, MSU-DOE Plant Research Laboratory

Two light protection functions: the OCP

Scientists have known for a while that most cyanobacteria contain a unique protein, the Orange Carotenoid Protein (OCP).  

Each OCP carries a carotenoid pigment in order to perform two crucial protective functions.

First, the OCP protects cyanobacteria overexposed to bright light by dissipating the excess absorbed light as heat. This “sunscreen” function prevents the photosynthetic machinery from burning out.

Second, the OCP scavenges harmful byproducts of photosynthesis that would otherwise damage the same photosynthetic system that produced them. (We eat leafy greens and drink orange juice to protect our bodies from such molecules).

Introducing the ancestor: HCP

The Kerfeld Lab - headed by Dr. Cheryl Kerfeld, Hannah Distinguished Professor of Structural Bioengineering - has now identified a new protein family they say were the OCP’s progenitors. These ancestors, named Helical Carotenoid Proteins (HCP) because of their coiled helix structures, consist of nine different protein sub-groups.

Image of coiled helixes
Coiled helix shapes (orange).
By Matt Melnicki, MSU-DOE Plant Research Laboratory

Modern cyanobacteria still contain genetic codes for the HCP family, which was how the Kerfeld Lab was able to discover it. “It turns out that the majority of cyanobacteria have at least one HCP or the OCP, or both, so there must be something important about them. We’ve uncovered a large diversity among the HCPs and we believe that the nine HCP sub-types each do something different.”

In fact, the HCPs look very similar to the OCP, and the research hints at which particular type of HCP evolved into the modern OCP.  “At some point – potentially a couple billion years ago – one of the HCPs acquired the ability to dissipate excess light, which provided an important fitness advantage.”

Cyanobacteria can’t run away or hide when light gets too bright, Matt adds. So acquring that sunscreen function allowed these organisms to venture out into riskier habitats where they had better access to light - such as those without shade, high UV exposure, or other stressful areas where photosynthesis doesn’t work at full capacity.

The Kerfeld lab thinks the HCP family may have additional functions distinct from the OCP’s, explaining why these ancestors have remained needed throughout the eons.

Image of a cyanobacteria bloom on Lake Atitlán, Guatemala, as seen from space.
The green swirls represent a cyanobacteria bloom in Lake Atitlán, Guatemala
By NASA Earth Observatory

Towards "light" applications

With increased understanding of these simple proteins, the Kerfeld lab are turning their attention to applied fields like medicine or biotechnology.

“The majority of known pigment-binding proteins are either stuck in lipid membranes in the cell – which makes them hard to work with – or are otherwise large and complex. The discovery of these HCPs, especially their diversity and solubility in water, opens new doors.”

One area of interest is optogenetics, a recently developed technology that uses light to control specific cells in living tissues (watch this great introduction below). Optogenetics is teaching us about how we wake up or how we learn. It has transformed the stumbling of rats afflicted with Parkinson’s into a steady walk and has led to insights into autism and depression.

In this context, the OCP and HCP, both responsive to light, could be used as switches that, when targeted by a light source, turn on predetermined functions – such as activating a neural pathway so scientists can observe it or curing a disease.

Medical applications could involve HCPs or OCPs transporting carotenoids – which are antioxidants – to pinpoint areas of oxidative damage in the human body or to combat diseases such as cancer. “We could brainstorm all day about what we could do with these things! Essentially, the limit will be our own creativity!”

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