Structure of the Azotobacter vinelandii surface layer

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• The Azotobacter vinelandii surface array is a proteinaceous layer crucial for cell structure.
• Understanding the assembly and stability of this S layer is important for bacterial cell biology.

Purpose of the Study:

• To elucidate the structure and assembly properties of the Azotobacter vinelandii S layer protein.
• To investigate the role of divalent cations in S protein stability and reassembly.

Main Methods:

• Electron microscopy and image processing to determine the S layer structure.
• Gel filtration to estimate the molecular weight of the S protein multimer.
• Circular dichroism spectroscopy to analyze protein secondary structure.
• In vitro reassembly assays to study S layer formation.

Main Results:

• The Azotobacter vinelandii surface array exhibits p4 symmetry with a unit cell of 18.4 nm.
• The S layer is composed of a 60,000 molecular weight protein that forms a stable tetrameric multimer.
• Tetrameric S protein dissociates into monomers in standard buffers but is stabilized by Ca2+ or Mg2+.
• Only the tetrameric form of S protein can reassemble in vitro in the presence of divalent cations.

Conclusions:

• The tetrameric form of S protein is the morphological subunit of the Azotobacter vinelandii surface array.
• Divalent cations are critical for stabilizing the S protein tetramer and enabling in vitro reassembly.
• These findings provide insights into the structural organization and assembly mechanisms of bacterial surface layers.