LONG EXPLANATION:
At upper right, an influenza virus particle is shown landing on
the cell surface. The virus docks with cell membrane when the red spikes (haemagglutinin)
link to molecules on the cell surface. The cell surface folds inwards causing the
virus particle to sink into the cell. The virus sinks deeper into the cell until
it is completely wrapped up in cell membrane. The resulting membranous "bubble"
(or vesicle) breaks free from the surface of the cell and transports its contained
virus into the cell. The netlike structure beneath the docking virus and the whitish
halo around the resultant vesicle represent clathrin, a protein that forms an external
scaffold that causes the cell membrane to invaginate and finally form the vesicle.
The clathrin coat is then lost and the virus in its naked vesicle can be seen half
out of frame at the right of the image. The engulfed virus then appears in an endosome
(the large irregular yellow vesicle at middle right). It is more acidic in the endosome
and this modifies the haemagglutinin spikes. The altered haemagglutinin draws the
membranes of the virus and endosome together and they merge, creating a hole through
which the viral contents are poured into the cytoplasm. These contents include the
viral matrix protein,
M1, (purple) and the nucleocapsid (green helix). Some
matrix protein is shown travelling to the
nucleus. The nucleocapsid segments,
which contain the viral genetic information, migrate to the nucleus. They move into
the nucleus via nuclear pores (the flower like structures on the curved surface of
the
nucleus) and so deliver the viral genome to the nucleus (which contains
the cell's own genetic material). In the nucleus, the viral genetic material (-ve
sense RNA) produces viral messenger RNAs of various kinds (
vmRNA) which travel
out through the nuclear pores. (Messenger RNA, or mRNA, carries the genetic information
that is used to direct protein maunfacture.) Some vmRNA directs the synthesis of
nucleoprotein (green dots) that travel back into the nucleus. Other vmRNA directs
the production of matrix protein (purple dots) shown emerging from a viral polyribosome
(several ribosomes strung together along a length of viral mRNA) in the middle of
the picture. Some matrix protein travels to the nucleus and some collects beneath
the cell membrane. Other vmRNAs direct the production of external (transmembrane)
viral proteins. The manufacture of such "external" proteins follows a different
route. Production starts in the rough endoplasmic reticulum and progresses through
the
Golgi apparatus. The haemagglutinin (red) is shown progressing through
the Golgi at lower left, finally being discharged onto the cell surface from a vesicle
(the sphere containing red dots that is delivering its contents onto the cell surface
through a hole). The neuraminidase (yellow) is shown (for clarity) going through
the Golgi in parallel but above the haemagglutinin. In the nucleus, the viral -ve
sense genome also produces full length +ve sense copies of itself. These are then
used to create further copies of the viral genome. These new -ve sense viral genomic
RNAs become associated with nucleoproteins and some matrix proteins that have migrated
into the nucleus. Such newly formed nucleocapsids and their associated M proteins
exit the nucleus via nuclear pores (the trail of green ribbons progressing across
the picture that exit from the lowermost nuclear pore). Just beneath the cell surface,
these individual ribonucleoprotein segments are shown associating together to form
the helical nucleocapsid (the green barrel-like structure). Around the new nucleocapsid,
the matrix proteins are shown collected beneath the cell membrane (the haze of purple
particles marked
M1), while above the cell membrane, haemagglutinin and neuraminidase
have coated the surface. With all these viral elements now in place, the newly forming
virus particle can begin to take shape and to bud from the cell surface. The cell
membrane that envelopes the emerging nucleocapsid and matrix protein becomes the
viral envelope (complete with projecting spikes) and the virus particle is released.
The new virus particle is now ready to infect another cell.
Although all these processes are shown occurring simultaneously they actually vary
with time and various mechanisms operate to regulate the phases of the life cycle.
The life cycle of influenza is rather complex and for more information, please check
the references below:
FOR INFORMATION ON VIRUSES ENTERING CELLS please see
virus
entry into animal cells from
Ed
Rybicki.
