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.
Bacteriophages must 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.
The T4 bacteriophage cell-puncturing device
consists of two proteins that 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. Each protein contains two types of subunits, each
consisting of proteins gp5 and gp27
. 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, gp5
narrows
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.
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GP27,
consisting of 319 amino acids, is a hollow cylinder approximately
80 angstroms
in diameter externally and 30 angstroms internally. GP27
consists
of four domains that form its barrel structure. The first
(residues 2-111)
and third
(residues
207-239
and 307-368)
are composed of seven or eight antiparallel beta sheets, creating a
torus
. The second
(residues 112-206)
and fourth (
residues 240-306
and 359-376)
domains bind the gp5 N-terminus domains through complementary polar and
charged residues with the surface of gp27 being mostly positively
charged
and the complimentary gp5 therefore having a negatively charged
surface.
The second and fourth domains are the main factor in the formation
of the
gp27 monomer-gp5 trimer complex. The dimensions of the gp27,
both
external and internal, are identical to the dimensions of the top
of the
tail tube, suggesting that gp27 acts as an extension of the gp5
tail tube,
connecting the baseplate and head of the phage to the tail tube (2).
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gp5 OB Fold Domain: The oligosaccharide binding domain facilitates the binding of oligosaccharides of the periplasmic peptidoglycan layer. This allows for the subsequent digestion of the host cell membrane by the lysozyme domain. This binding function suggests that gp5 may separate from the baseplate and head of the phage during tail contraction (2).
gp5 Lysozome Domain: The lysozyme domain, composed of three symmetric units is located between the OB domain and the Beta Helix domain. The lysozyme domain binds to the peptides of the host cell membrane and degrade the membrane proteins, making it easier for the needle to puncture the cell membrane. When correctly aligned, Pro 363, Ala 364, and Asp 365 were shown to directly bind peptidoglycan substrate suggesting that it would also bind the E. coli periplasmic cell wall to allow for the membrane proteins to be degraded (2).
gp5C Beta-helix Domain:
The beta helix consists of three polypeptide chains that wind to
form an
equilateral triangular prism with a left-handed twist. Due to
the
narrowing of the external side chains and the presence of internal methionine
554 and methionine
557
,
the helix narrows from 33 to 25 angstroms, forming a needle.
Glycine
is replaced by Asp
436
to form the initial helix structure, which is further stabilized by
hydrogen
bonding to the lysozyme domain. The structural stability of
the helix
comes from two ions: a phosphate ion
coordinated by three
Lys 454
residues and a potassium ion
coordinated by three Glu
552
(2).
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Bacteriophages have contributed immensely to scientific studies in genetics. Hershey and Chase (1952) conducted a set of inheritance experiments known as the "blender experiments." Their results proved that DNA is responsible for genetic inheritance. Using T2 bacteriophages, which also attack E. coli, Hershey and Chase were able to grow colonies of radiolabelled phages. Viral DNA was tagged with radiolabelled phosphorous (32P) and viral proteins were tagged with radiolabelled sulfur (35S). DNA-labelled and protein-labelled phages were allowed to infect separate colonies of E. coli cells. The samples were "shaken" to release any phages that were still attached to the outside of the host cell and were centrifuged so that cellular debris would form a pellet. The pellets could then be examined for levels of radioactivity.
Upon statistical analysis they found that protein-labelled material was mainly localized to the supernatant with the viruses and that DNA-labelled material was localized to the cellular pellet. They then allowed the radio-labelled phage progeny to reproduce. They found that the progeny of the protein-labelled phages did not possess any radiolabelling, however, DNA-labelled phage progeny had radiolabelled phosphorous in their DNA.
From this data they were able to conclude that the protein shell of the bacteriophage remained outside of the host cell while the genetic material was transplanted into the host organism, where viral enzymes promoted bacteriophage replication. It wasn't until later that scientist discovered that DNA was injected into the host cell via an intricate cell-puncturing device.
2. Kanamaru, S., Petr G. Leiman, Victor A.
Kostyuchenko,
Paul R. Chipman, Vadim V. Mesyanzhinov, Fumio Arisaka, and
Michael
G. Rosmann. 2002. "Structure of the cell-puncturing
device
of bacteriophage T4." Nature. 415: 553-557.
3. Kostyuchenko, Victor A., Grigorii A. Navruzbekov, Lidia P.
Kurochinka,
Sergei V. Strelkov, Vadim V Mesyanzhinov, and Michael G.
Rossman.
1999. "The structure of bacteriophage T4 gene product 9: the
trigger
for tail contraction." Structure. 7: 1213-1222.
4. Leiman, Petr G., Victor a. Kostyuchenko,
Mikhail
M. Shneider, Lidia P. Kurochkina, Vadim V. Mesyanzhinov, &
Michael
G. Rossmann. 2000. "Structure of Bacteriophage T4 Gene
Product
11, the Interface Between the Baseplate and Short Tail
Fibers." Journal
of Molecular Biology. 301, 975-985.