Team:MIT phage background


hairy cells and polymerizing phage - background
Arap, 2005
M13 bacteriophage is a long (930 nm in length, 6.5 nm in diameter), filamentous virus that infects bacteria via interactions with the F-pilus. Below is the list of genes that comprise the phage:

  • pIII/pVI (pointed end) – mediate termination of assembly and particle release from the membrane; is present at around five copies
  • pVII/pIX (blunt end) – small coat proteins; is present at around five copies
  • pVIII – major coat protein; is present at around 2700 copies
  • pII – nicking enzyme, allows for replication to occur
  • pX – last third of PII; may inhibit PII in a regulatory manner
  • pIV – channel for phage exit
  • pI – may hydrolyze ATP to promote assembly
  • pXI – last third of PI, may interact with PI to form a channel that works with PIV
  • pV – dimerizes and binds ssDNA, packaging it into a rod for assembly

Assembly of M13 happens through a three-part process:

  • Initiation: pVII/pIX and pVII interaction with the DNA packaging signal. This is mediated by PI.
  • Elongation: DNA is extruded with pV being replaced by pVIII. This is mediated by pI. The particle elongates through the pIV channel.
  • Termination: pIII/pXI are added to the end of the particle. When pIII is not present due to a knockout, elongation proceeds through multiple phage genomes, resulting in particles that are tethered to the membrane and up to 20x normal length.

Barbas III et al., 2004

The goal of phage display is to use M13 to carry a foreign protein or peptide on its surface, and package the DNA corresponding to that foreign peptide. One accomplishes this by genetically fusing a the peptide sequence of interest to one of the coat proteins. This modification can be genomic, in which case all copies of the coat protein will display the peptide, or the fusion can be expressed on a separate plasmid. In the separate plasmid (phagemid) system, the fusion proteins are integrated along with the wild type coat proteins.

Historically, phage display has been used as a mechanism for screening libraries of proteins or peptides for binding to a specific substrate or ligand of interest. In a process called panning, a population of phage is iteratively enriched for those that bind a substrate. Phage with proteins that do not bind are washed away and the remaining phage can then be amplified through infection.

We use principles of phage display only for the display property (rather than the packaging property). We also use a phagemid system. See Design page for details.

Coiled coils are a common structural motif found in proteins. Repetitive amino acid sequences in a pair of alpha-helices provides a mechanism for the helices to interact with each other, coiling around each other. Leucine zippers are of this class--every seventh residue is a leucine. The leucines and other hydrophobic residues form a hydrophobic core, allowing for the helices to "zip" together. In our project we're using two leucine zipper pairs: Fos/Jun and ACID/BASE, and one pair of modified coiled coils: GR1/GR2. For more information on coiled coils, see Branden and Tooze (1999).

All these pairs of coils have been successfully displayed on M13, and in the case of Fos/Jun and ACID/BASE cross-linking of phage has been observed (Wang et al. 2010, Sweeney et al. 2007).

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