The filamentous phage pIV multimer visualized by scanning transmission electron microscopy

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Secretins are homomultimeric outer-membrane proteins essential for bacterial secretion.
• They transport large macromolecules like enzymes and filamentous bacteriophages.
• Understanding secretin structure is key to deciphering secretion mechanisms.

Purpose of the Study:

• To characterize the structure and assembly of the secretin from filamentous phage f1.
• To determine the size and stoichiometry of the secretin multimer.
• To assess the potential of the secretin channel to accommodate phage particles.

Main Methods:

• Purification of the filamentous phage f1 secretin.
• Mass determination using scanning transmission electron microscopy (STEM).
• Analysis of the protein's cylindrical structure and internal diameter.

Main Results:

• The f1 phage secretin was purified and analyzed.
• Two distinct multimeric forms were identified: a unit multimer (~14 subunits) and a multimer dimer.
• The secretin forms a cylindrical channel with an ~80 angstrom internal diameter.
• This diameter is sufficient to accommodate filamentous phage (~65 angstroms).

Conclusions:

• The f1 phage secretin exists as stable multimers capable of forming a large translocation channel.
• The structural data supports the role of this secretin in filamentous phage export.
• Secretins represent a crucial class of protein transporters in bacteria.