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Structure and stability of protein H and the M1 protein from Streptococcus pyogenes. Implications for other surface
B H Nilson1, I M Frick, P Akesson
1Centre for Protein Engineering, MRC Centre, Cambridge, England.
Abstract:
M proteins and other members of the M protein family, expressed on the surface of Streptococcus pyogenes, bind host proteins such as immunoglobulins, albumin, and fibrinogen. Protein H and the M1 protein are expressed by adjacent genes and both belong to the M protein family. In this work, the structure and stability of these two proteins have been investigated. As judged from sequence analysis and circular dichroism spectroscopy, the proteins are almost entirely in an alpha-helix conformation. The amino acids are arranged in a seven-residue (heptad) repeat pattern along the greater part of the proteins. These observations support the previously accepted model of M proteins as coiled-coil dimers. However, it was also found that the structures of both proteins were thermally unstable; i.e., the content of helix conformation was greatly reduced at 37 degrees C as compared to 25 degrees C or below. Together with previous findings that these proteins appear as monomers at 37 degrees C and dimers at low temperatures, the results suggest that the coiled-coil dimers are unfolded at 37 degrees C. The heptad patterns of protein H and the M1 protein showed a nonoptimal distribution of residues expected for a coiled-coil conformation. This is a possible explanation for the low thermal stability of the proteins. It was also demonstrated that the proteins were stabilized in the presence of the ligands IgG and/or albumin. Protein H and M1 protein show a high degree of sequence similarity in their C-terminal regions, and a fragment from this region displayed a high content of helix conformation, whereas fragments from the nonsimilar N-terminal parts did not adopt any stable folded structure. Thus, the C-terminal parts, which are conserved within the M protein family, may constitute a framework for the formation of the parallel helical coiled-coil structure, and we propose that the less stable N-terminal part may also participate in antiparallel interaction with M proteins on adjacent bacteria. The results suggest that temperature fluctuations in the environment could change the properties of bacterial surface proteins, thereby affecting the molecular interactions between the bacterium and its host.
Insights
Streptococcus pyogenes M proteins, like Protein H and M1, are thermally unstable coiled-coil dimers that unfold at body temperature. Ligands like IgG stabilize their structure, suggesting temperature impacts bacterial interactions.
Area of Science:
- Microbiology and Molecular Biology
- Protein Structure and Stability
- Bacterial Pathogenesis
Background:
- M proteins on Streptococcus pyogenes surfaces bind host proteins, aiding bacterial evasion.
- Protein H and M1 protein are M protein family members with potential roles in host interaction.
Purpose of the Study:
- To investigate the structure and thermal stability of Protein H and M1 protein.
- To understand the implications of their structural dynamics for bacterial-host interactions.
Main Methods:
- Sequence analysis and circular dichroism spectroscopy to determine protein structure.
- Thermal stability assays at different temperatures (25°C vs. 37°C).
- Ligand binding studies with IgG and albumin.
Main Results:
- Both proteins exhibit primarily alpha-helix conformation, forming coiled-coil dimers at lower temperatures.
- Structures are thermally unstable, unfolding into monomers at 37°C, with nonoptimal heptad repeats.
- Ligands (IgG, albumin) stabilize the protein structures; conserved C-terminal regions are crucial for helical structure.
Conclusions:
- The coiled-coil dimers of Protein H and M1 protein are unstable at physiological temperatures.
- Structural instability may be linked to nonoptimal residue distribution, impacting bacterial interactions.
- Temperature fluctuations could modulate bacterial surface protein properties and host interactions.