A trans-envelope protein complex needed for filamentous phage assembly and export.

Area of Science:

• Molecular biology
• Virology
• Membrane protein complex

Background:

• Filamentous phage assembly and export rely on specific non-capsid proteins.
• Four key proteins are essential: pIV (outer membrane), pI and pXI (inner membrane), and thioredoxin (cytoplasmic host factor).

Purpose of the Study:

• To investigate the early molecular interactions and complex formation during filamentous phage assembly.
• To identify the initial protein interactions that constitute the preinitiation complex.

Main Methods:

• Chemical cross-linking of intact bacterial cells.
• Protease protection assays to assess protein stability and complex formation.

Main Results:

• A trans-membrane complex containing inner membrane proteins pI and pIV was identified.
• Formation of this pI-pIV complex conferred protection to pI against endogenous protease degradation.
• The presence of pXI, identical to pI's C-terminus, was also required for pI protection and is part of the complex.
• This complex assembles independently of phage DNA and other phage proteins, signifying a preinitiation step.

Conclusions:

• The pI-pIV-pXI complex represents the initial preinitiation stage of filamentous phage morphogenesis.
• Conversion to an initiation complex likely occurs upon binding of phage DNA, thioredoxin, and minor coat proteins.