Structure of outer membrane protein G in lipid bilayers

Outer membrane proteins in bacteria play key roles in nutrient transport and cell signaling. These proteins often form β-barrel structures that span the membrane. While prior research has described general features of β-barrel proteins, the precise structural dynamics of these proteins in native-like lipid environments remain unclear. X-ray crystallography and solution NMR have provided conflicting models for outer membrane protein G (OmpG). This gap motivated researchers to use a more biologically relevant lipid bilayer system to investigate OmpG’s structure. The extracellular loops of OmpG are suspected to influence its function, but their exact roles remain unresolved. No prior work had resolved the structural differences between strands and loops in the membrane-bound state. Understanding these differences could clarify how OmpG functions in bacterial physiology. This study aimed to provide a high-resolution structural model of OmpG in a lipid bilayer context.

The goal of this research was to determine the structure of outer membrane protein G (OmpG) in a lipid bilayer system that mimics its native environment. OmpG is a 14-strand β-barrel protein in Escherichia coli, and its function is thought to involve pH-dependent opening and closing. Previous structural studies have yielded conflicting results, particularly regarding the extracellular loops. This uncertainty drove the need for a more accurate structural model. The researchers sought to examine how the protein’s structure changes in a membrane-like setting. They aimed to clarify the roles of specific β-strands and loops in maintaining the protein’s stability and function. By using a lipid bilayer system, the study aimed to capture the protein’s behavior in a biologically relevant context. The findings could help explain how OmpG contributes to bacterial survival and function.

The researchers used magic-angle-spinning NMR to study the structure of outer membrane protein G in a lipid bilayer system. The bilayer was composed of E. coli lipid extracts, which closely resemble the native membrane environment. The study utilized 1847 inter-residue hydrogen-hydrogen and carbon-carbon distance restraints to calculate the protein’s structure. Torsion angles were also measured, totaling 256 data points. No hydrogen bond restraints were used in the structural calculations. The protein was analyzed in a membrane-bound state to capture its natural conformation. The NMR data were used to determine the spatial arrangement of β-strands and extracellular loops. The method allowed for high-resolution structural insights without relying on crystallographic assumptions.

The structure of outer membrane protein G revealed that β-strands vary in length beyond the membrane boundary. Strands 6 through 8 were found to be the longest in the structure. Extracellular loops 3 and 4 were well ordered, suggesting a functional role in the protein’s stability. The site of barrel closure at strands 1 and 14 was more disordered compared to other regions. This disorder decreased toward loops 3 and 4, indicating a structural gradient. Loop 4 was found to contain a well-defined helix, which may contribute to the protein’s function. The study provided a detailed structural model of OmpG in a membrane-like environment. These findings suggest that the extracellular loops play a significant role in maintaining the protein’s conformation.

The authors suggest that the structural model of outer membrane protein G in a lipid bilayer provides insights into its function in Escherichia coli. The findings indicate that the extracellular loops, particularly loops 3 and 4, are well ordered and may contribute to the protein’s stability. The disordered region at strands 1 and 14 implies flexibility in the barrel closure site. The presence of a helix in loop 4 supports the idea that this region is functionally important. The study confirms that β-strand lengths vary beyond the membrane boundary. These structural features may influence the pH-dependent opening and closing of the protein. The results do not suggest a need for additional hydrogen bond restraints in future studies. The model supports the hypothesis that OmpG’s structure is adapted to its membrane environment.