Genetic defects reveal details of the protein factory

Some time ago, Utrecht University researcher Friedrich Förster received cells from two patients with rare genetic defects. These patients lacked different parts of a protein complex, of which the precise function is largely unknown. By identifying the structure of this protein the Professor of Cryo-Electron Microscopy now reveals more about the functions of its components. Together with his colleagues at the Max Planck Institute of Biochemistry in Munich, the Saarland University in Homburg  and the Sandford Burnham Prebys Medical Discovery Institute in San Diego (La Jolla) he published the findings in Nature Communications on February 20.

Förster is specialised in research of membrane-bound proteins located in the cell’s protein factory, the endoplasmic reticulum. Around one third of the production of proteins occurs there. In the endoplasmic reticulum, the proteins are folded correctly and sugar groups are added in a process called ‘glycosylation’, so that they can properly perform their function in the body.

A major element in this process is the translocon, a massive protein complex in the membrane of the endoplasmic reticulum. This complex consists of a channel through which proteins can pass, and a part that binds sugar molecules to proteins. However, until now scientists had not unravelled the detailed function of the third and last part of the complex, called TRAP (translocon-associated protein complex). “But we know it’s there”, says Förster. “And we know it’s a fixed element of the translocon, so it must be important.” 

Teenagers with genetic defects

When a pediatrician from San Diego approached him, Förster was immediately enthusiastic.  The doctor, Hudson Freeze, was treating two teenagers with a mental handicap and a serious developmental disability. Genetic analysis had shown that both children had different mutations in the genes that code for the TRAP complex. It was also known that the sugar groups were missing on some of the proteins in these patients. Förster: “This indicates that TRAP is necessary for glycosylation.”

Förster, who recently received a major European grant for his research into the endoplasmic reticulum, decided to find out how the teenagers’ TRAP complex deviated from the norm at the molecular level. To do so, he compared the structure of TRAP in these patients to that of a healthy human cell culture using cryo-electron tomography, an emerging technique for imaging the molecular structure of large proteins in their natural environment with a high degree of detail.

Two different functions

“These patients both lacked one of the four subunits that make up TRAP,” Förster explains. “The first patient lacked the delta subunit. The images from the electron microscope showed that in healthy people, this subunit is located near the complex of the translocon that is important in glycosylation. From that, we concluded that the TRAP delta unit supports the glycosylation process.”

The other patient was missing the gamma subunit of TRAP. The structural analysis showed that as a result, the entire protein can no longer bind to the translocon. Förster: “It’s still inside the cell, but it’s not in the place it is supposed to be. That means the gamma unit ensures that TRAP stays in the proper position.”

Algae

In order to further clarify the TRAP subunit functions, Förster and his colleagues examined the endoplasmic reticulum in algae. In these organisms, TRAP consists of only two elements. And it is exactly the two elements that the patients lack that are also missing in algae. Förster: “For our initial research, we had to break open the cells from the patients, but with the algae we could look inside a complete cell, leaving the molecules completely unperturbed. The image was consistent with that of the two patients. As a result, we could confirm the position of the two missing subunits.”

With his work, Förster hopes to gain a better understanding of the molecular bases of diseases involving problems with protein production and maintenance at the endoplasmic reticulum. These include not only this rare genetic defect, but also diseases such as cystic fibrosis and neurodegenerative conditions such as Alzheimer’s disease.