T4 Bacteriophage Cell-Puncturing Device
Clint Priestley '03 and Matt Schefft '04
bacteriophage consists of a head, phage tail, and baseplate.
The head contains double stranded viral DNA, which is ejected into host
cells in order to propagate the viral infection (2).
The tail connects to the baseplate, which attaches to long and short tail
fibers responsible for recognizing host cells and then anchoring the phage
to the host cell (4). The tail also contains the
cell-puncturing device that is instrumental in cell puncturing and
injection of the phage DNA into the host cell (3). The
T4 bacterophage is an important tool in research as well as a good system
of study. T4 bacteriophages are specific to E. coli so they
remain dormant virions until their tail fibers come in contact with a binding
site on an E. Coli cell wall. This makes T4 an excellent vehicle
for the insertion of a specific DNA sequence or vector.
T4 bacteriophages reproduce via a lytic
life cycle. Without their cell-puncturing device T4 bacteriophages
would be unable to introduce their DNA into the cell of a host system.
The lytic cycle allows the T4 bacteriophage to transform a host cell into
a replication machine. Phages exhibit chracteristics of living cells,
such as the ability to mutate and to reproduce astonishingly quickly (in
a living cell). However, they also exhibit non-living cell traits.
Phages are acellular and thus unable to metabolize or replicate without
the aid of the metabolic machinery of their host bacterium. Also,
they can possess either DNA or RNA but not both. The
lytic life cycle observed in T4 bacteriophages is another example of
nature's wonderful evolutionary capabilities.
first bind to the bacteria cell wall in a process called adsorbtion in
order to begin the lytic cycle. The phage then penetrates the bacteria
cell wall using its sheath and then injects its genetic material into the
host via flagella. Phage enzymes shut down the cells own DNA and
RNA synthesizing pathway and replicates its own genome. Bacteriophage
components begin to be produced by way of the host bacterium's metabolic
machinery. The mature phage components then begin to assemble themselves
around the genome, encapsulating it in the head region. The newly
formed phages are ready to lyse from the host cell and infect new cells.
A bacteriophage enzyme breaks down the bacterial peptidoglycan causing
osmostic lysis. This process can produce 50-200 new phages that will
spread throughout the host system infecting and destroying cells.
Throughout this site you may view either the
Trimeric form of the heterononameric assembly (gp27-gp5-gp5C) or a single
monotrimer of the structure by toggling the links in the main frame.
You will notice how each monotrimer interacts with the others to form a
fully functional and extremely efficient structure.
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II. General Structure
The T4 bacteriophage cell-puncturing device consists
of two proteins, gp5
and gp27 .
They form a stable complex which makes up the hub of the T4 baseplate.
The hub is surrounded by known proteins such as gp9 (3)
gp10 (3), and gp11 (4)
as well as others yet to be studied. The cell-puncturing device resembles
a torch, such that gp5 forms the handle, and the flames would come from
gp27. At the C-terminal end, gp5narrows
to form a needle
by which the cell membrane can be punctured. This needle is comprised
of beta-helical domain, which is also known as gp5C. This process
is aided by the lysozyme region of the gp5 domain responsible for digesting
the intermembrane peptidoglycan layer of the cell as the needle contracts.
T4 is also composed of three trimeric gp5-gp27 units. Each unit contains
a third of the barrel shaped gp27, one of the three lysozyme subunits,
and one of the three polypeptide chains which make up the helix needle.