Description of Paramyxoviridae
adapted from
ICTVdb
Virion Properties
Morphology
Virions consist of an
envelope, a nucleocapsid, and a matrix protein. During their life
cycle, virions have an extracellular phase. Virus capsid is
enveloped and mature naturally by budding through the membrane of
the host cell; spherical to pleomorphic; filamentous and other
forms are common. Virions measure (60-)150-200 nm in diameter;
1000-10000 nm in length. The envelope has surface projections.
Surface projections are spaced widely apart, distinctive spikes
of haemagglutinin-neuraminidase (HN) and fusion (F) glycoproteins
covering evenly the surface; embedded in a lipid bilayer which is
comprises hemagglutinin and neuraminidase (HN), or hemagglutinin
(H), or surface glycoproteins (GP), or fusion proteins. Surface
projections are homo-oligomers and form spike-like projections of
8-12 nm long; spaced 6-10 nm apart (depending on the genus).
Capsid/nucleocapsid is elongated and exhibits helical symmetry.
The nucleocapsid is filamentous, flexuous with a varying length
with a length of 600-800(-1000) nm (depending on the genus) and a
width of 13-18 nm. Basic helix is obvious. Pitch of helix is
5.5-7 nm (depending on the subfamily). The nucleocapsid is not
segmented.
Physicochemical and Physical Properties
The molecular mass (Mr) of
virions is 500 x 10
6 (occassionally, multiploid
virions are found with a greater Mr). Virions have a buoyant
density in CsCl of 1.18-1.31 g cm
-3; sucrose of
1.18-1.2 g cm
-3. The sedimentation coefficient is at
least 1000
S20w. Virions are sensitive to
treatment with lipid solvents, non-ionic detergents,
formaldehyde, oxidizing agents, heat (to the extreme).
Nucleic Acid
The Mr of the genome constitutes 0.5% of the virion
by weight (; the Mr of the genome is 5-7 x 10
6). The
genome is usually monomeric, or multiploid (sometimes, not
segmented and contains a single molecule of linear
negative-sense, single-stranded RNA. The genome is not infectious
(by itself). Virions may also contain occasionally a positive
sense single-stranded copy of the genome (thus, partial
self-annealing of extracted RNA may occur). The complete genome
is 15200-15900 nucleotides long. The RNA is sequenced and
complete sequence is about 15200-15900 nucleotides long and
encodes 7-9/9-11, namely three nucleocapsid-associated proteins
(N or NP; P and L, three membrane-associated proteins, a matrix
protein M and two envelope proteins (F and G, or H, or HN). The
5'-end of the negative-sense strand does not have a covalently
attached terminal protein; genome does not have cap. The
3'-terminus has no poly (A) tract. Each virion contains a single
copy of the genome; a full length copy (of RNA exclusively as
nucleocapsid, or intracellularly).
Proteins
Proteins constitute about about
75-80% of the particle weight.
The viral genome encodes structural proteins and
non-structural proteins. Virions consist of 6-7 structural
protein(s) located in the nucleocapsid, envelope, membrane, and
matrix. The viral envelope contains 2 integral membrane
proteins.
Structural Proteins: Envelope protein F.
Envelope protein has a function assigned; is a fusion protein.
During post-translational processing envelope protein F is
synthesizied within an infected cell, has been cleaved from the
precursor protein (by cellular protease(s) to produce the virion
disulfide-linked F1 and F2 subunits (amino
F2-S-S-F1 carboxyl), during
post-translational processing envelope protein modifications
occur that include glycosylation, or disulfide cross-linking.
Envelope protein G (Pneumovirus), or H
(Morbilivirus), or HN (Paramyxovirus, has been
sequenced, or a function assigned; is an attachment protein which
possess(es) hemagglutination activity in all genera except
Pneumovirus where it is called G protein. If the protein
also contains neuraminidae activity as in the genus
Paramyxovirus it is called HN; during post-translational
processing envelope protein modifications occur that include
glycosylation. Envelope protein has a molecular mass of 22000 Da.
Envelope protein SH or A; is a small integral a membrane protein.
Nucleocapsid protein N or NP has a molecular mass of about 50000
Da; is protecting of the genome; which possess(es) RNA-binding
activity. Nucleocapsid protein P is found in the virion in about
10-fold greater abundance than protein L; has a molecular mass of
40000-60000 Da; is polymerase associated and has has possibly a
template melting function; modifications during
post-translational processes include phosphorylation.
Nucleocapsid protein L is large and; is a putative polymerase;
which possess(es) RNA catalyzing activity in connection with
protein P. Matrix protein M is an unglycosylated inner membrane
protein.
Non-Structural Proteins: Virus-coded
non-structural proteins have been identified by sequence analysis
and at least 6 non-structural protein(s) are found. The virus
codes for enzymes. In addition to the polymerase, the virus codes
for enzymes such as found variously expressed amomg the genera
and include a RNA-dependent RNA transcriptase, adenylated
transferase, mRNA guanylyl transferase, methyl-transferase,
proteinase, and neuraminidase. The non-structural protein is
associated with the membrane protein. Non-structural protein C.
Non-structural protein NS1. Non-structural protein NS2.
Non-structural protein V, a cystein rich protein,. Its role is
zinc binding. Non-structural protein SH, a small integral
membrane protein. Non-structural protein M2, formerly called
22-kDa. Its role is transcription processivity factor, which
previously was thought to be a second M-like protein.
Lipids
Lipids are present and located in the
envelope. Virions are composed of 20-25% lipids by weight. The
composition of viral lipids and host cell membranes are similar.
The lipids are of host origin and are derived from plasma
membranes.
Carbohydrates
Carbohydrates are found in
virions; constitute 6% of virion dry weight; are present as
glycoproteins; are N-linked glycans (side chains found in the
fusion and attachment proteins), or O-linked glycosidic side
chains (in the attachment protein G in the subfamily
Pneumoviridae) and contain polylactosamine (in the SH
protein of
Respiratory syncytical virus). Carbohydrate
composition in the virion is host-dependent.
Genome Organization and Replication
Virions attach to specific receptors located on the surface
of cell membrane and enter host cells via fusion of the viral
envelope with the host cell surface in an environment of neutral
pH.
Transcription: The viral
genome is transcribed processively from the 3' end by
virion-associated enzymes.
The viral genome is transcribed by a viral associated
enzymescanal is distinct; to 5-10 mRNA(s). The transcribed mRNAs
are subgenomic in a viral-complementary sense. Transcription is
guided by short (10-13 bp) conserved transcription start and
termination/polyadenylation signals flanking each transcriptional
element).
The 5' ends of mRNAs are capped. The 3' ends of mRNAs possess
a poly (A) tract (synthesized by reiterative copying of the
polyadenylation site). Intergenic regions of viral genome may
vary in size and sequence between genera ((Rubulavirus,
Pneumovirus) or are highly conserved in sequence and
length (Respirovirus, Morbilivirus)).
Translation: The genome
replicates in the cytoplasm. The parental genome does not serve
as template. Replication is independent of host nuclear
functions.
Replication cycle
Accumulations of virions in vitro are sensitive to
amantadine.
Assembly and Egress:
Independently nucleocapsid.
Maturation: The mature
virus is found in the respiratory tract; skin. In thin sections
the mature virus is seen in crystalline arrays. Nucleocapsids are
enveloped at sites containing virus capsid proteins.