Bacteriophage SPP1 tail tube protein self-assembles into β-structure-rich tubes

Area of Science:

• Structural biology
• Virology
• Molecular biology

Background:

• Bacteriophages utilize long tails for host recognition and DNA injection.
• Tail tubes, primarily composed of tail tube proteins (TTPs), form a conduit from the capsid to the bacterial cell.
• In contractile phages, a sheath surrounds the TTP tube.

Purpose of the Study:

• To investigate the self-assembly properties of the siphophage SPP1 tail tube protein, gp17.1.
• To determine the structural characteristics of gp17.1 during and after tube formation.
• To elucidate the role of specific gp17.1 structural elements in assembly.

Main Methods:

• Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy to analyze protein structure.
• Electron microscopy (EM) to visualize assembled tubes.
• In vitro assembly assays and in vivo phage particle morphogenesis studies.

Main Results:

• SPP1 phage gp17.1 self-assembles into tubes without other viral proteins.
• gp17.1 monomers lack stable structure but gain a β-sandwich fold upon assembly, confirmed by FTIR and NMR.
• EM revealed tubes composed of helically stacked hexameric rings, mirroring native SPP1 virion tails.
• A specific loop (residues 40-56) is crucial for in vitro tube assembly and in vivo tail formation.

Conclusions:

• SPP1 phage gp17.1 is capable of self-assembly into functional tail tubes.
• Protein structure stabilization is coupled to the self-assembly process.
• The identified loop is essential for both in vitro and in vivo assembly, highlighting its critical role in phage morphogenesis.