Deciphering the Protein-Transport Mechanism of Cells

Originally Published MDDI September 2002

R&D DIGEST

As medical technology enters a postgenomic era, gaining a better understanding of the structure and function of the proteins found within cell membranes remains an imposing challenge. Achieving this knowledge, however, could ultimately lead to improved diagnoses of various neurogenerative diseases, such as Alzheimer's, as well as certain inherited and cardiac diseases.

Among the functions that some researchers believe could be key to understanding cell processes is protein transport, which is a fundamental mechanism in all cells. In this process, certain proteins are secreted or targeted to a specific compartment by membrane translocation or insertion. Now, researchers at the Max Planck Institute of Biophysics (Frankfurt, Germany) believe they have identified the structure of what could be considered a protein-transport machine.

Simple organisms such as bacteria are relatively simple cells with only one or two compartments, while higher organisms have cells with several different compartments. Yet the most significant protein-translocation system found in bacteria resembles one found in higher organisms, including humans. The researchers explain that bacteria such as Escherichia coli have a ubiquitous transport system known as the Sec pathway. Three different membrane proteins— SecY, SecE, and SecG—make up the core component, SecYEG.

The researchers say most E. coli proteins are transported by the SecYEG complex. The proteins typically contain a signal sequence, which acts like a post-code and is recognized by the transport machinery, they explain. The proteins are then transported through a channel formed by the SecYEG complex.

The group, led by Ian Collinson, PhD, has succeeded in determining the structure of the SecYEG protein-transport mechanism from E. coli. The researchers were able to grow two-dimensional crystals of the membrane-inserted complex, and then determined its structure using electron cryomicroscopy and image processing.

According to the research team, the structure provides the first detailed view of a protein-transporting machine. "The images show a dimer, or unit, of the SecYEG complex in its native environment, the lipid membrane," they report.

The researchers believe the dimer is the active form of the complex. They add that there are also elements suggestive of a secondary structure. As seen in the figure at left, at the dimer interface there is a cavity that may be the doorway through which proteins are pushed by associated partner proteins. The group believes identifying this structure represents the first step on the way towards an in-depth understanding of this fundamental biological process.

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