Module 10: Viral Rep. Strategies (Host Cell Recognition + Viral Entry/Uncoating) Flashcards
(94 cards)
1
Q
Viral replication is not uniform mainly due to…
A
Different types of viral genomes!
== Must employ different methods in order to replicate
2
Q
How are viruses “Obligate Intracellular Parasites”?
A
They only replicate in appropriate host cells!
3
Q
Viral Attachment Protein
A
Protein expressed on the surface of a virus that’s used for host cell attachment
4
Q
Receptor
A
A binding molecule present on the surface of a host cell
5
Q
What is the normal purpose of host cell receptors?
A
Often the host cell receptors have a function for regular cell activities like signaling and communication
6
Q
Host cell receptors get ___________ by viruses
What does this mean?
A
Host cell receptors get HIJACKED by viruses
== Viruses bind to host cell receptors allowing them to attach but preventing the normal function of the receptor!
7
Q
Host Range
A
Set of species/cells that can be infected by a given virus
8
Q
What determines viral host range?
A
The SPECIFICITY of viral attachment protein:receptor interaction
9
Q
What is the relationship between host range and specificity of viral attachment protein:receptor interaction?
A
As specificity increases, host range decreases
10
Q
What is an example of a non-enveloped virus that attaches to PM from protein extending from CAPSID?
A
Adenovirus
11
Q
What are the 4 main types of receptor:virus interactions (attachments)?
A
1) Bacteriophage tail fibers : Host cell receptors
2) Envelope Embedded Viral Attachment Proteins : Host cell receptors
3) NON-enveloped viral attachment proteins (on capsid) : Host cell receptors
4) NON-enveloped viral capsid : Host cell receptors
12
Q
What is the process of bacteriophage attachment?
A
1) Phage randomly “bumps” into a potential host cell
2) One tail receptor recognizes an appropriate receptor + virus becomes weakly bound to the cell
3) Remaining tail fibers bind to additional receptors = virus binds strongly to the host cell surface
13
Q
What is the viral attachment protein of H1N1 influenza?
A
HA (hemagglutanin)
14
Q
CD4
What is its normal function? What virus hijacks it?
A
CD4 = normally used for interactions with immune cells
–> Hijacked by HIV!
15
Q
ICAM-1
What is its normal function? What virus hijacks it?
A
ICAM-1 = Cell adhesion
–> Hijacked by Rhinovirus
16
Q
Bgp1a
What is its normal function? What virus hijacks it?
A
Bgp1a = Cell adhesion
–> Hijacked by MHV
17
Q
CR2
What is its normal function? What virus hijacks it?
A
CR2 = For B-cell activation
–> Hijacked by Herpesviridae (Epstein-Barr Virus)
18
Q
Sialic Acid
What is its normal function? What virus hijacks it?
A
Sialic Acid (SA) = has various functions
–> Hijacked by Influenza
19
Q
OmpF
What is its normal function? What virus hijacks it?
A
OmpF = Transmembrane Channel
–> Hijacked by T2 phage
20
Q
What are the 2 main methods of non-enveloped viral attachment?
A
1) Viral attachment proteins extend from capsid + interact with host cell receptors
2) Proteins of the viral CAPSID itself interact with the host cell receptors
21
Q
What receptors on E.coli K12 does the T2 phage recognize + bind to?
A
1) Lipopolysaccharide (LPS)
2) Outer Membrane Protein F (OmpF)
22
Q
How do phages T2 + PPO1 differ?
A
T2 = Can infect E.coli K12 BUT CANT infect O157:H7 E.coli strain
PPO1 = CANT infect E. coli K12 but CAN infect E. coli O157:H7!
23
Q
In the experiment of T2 phage attachment to K12 + O157:H7, what was the hypothesis?
A
T2 does not infect O157:H7 because of STRAIN-SPECIFIC differences in the receptors
24
Q
In the experiment of T2 phage attachment to K12 + O157:H7:
What test was conducted?
A
The T2 phage was genetically engineered to have two of its tail fibers proteins (gp37 + gp38) replaced with the corresponding proteins from PP01
== Generated the T2PPD1 phage
25
What are gp37 + gp38?
Proteins at the tips of the T2 tail fibers
26
T2PPD1 Phage
An altered T2 phage with PP01 tail fiber proteins
27
What were the results of the experiment of T2 phage attachment to K12 + O157:H7?
The altered T2 phage (T2PPD1) =
1) LOST its ability to bind to E. coli K12 BUT
2) GAINED an ability to bind to E. coli O157:H7!
== The altered phage exhibits an altered host range!
28
What conclusion was made from the results of the experiment of T2 phage attachment to K12 + O157:H7?
Conclusion = The interactions between viral attachment proteins and host cell receptors determine what strain T2 can infect!
== Host range is determined by phage attachment proteins!
29
MHV
Mouse Hepatitis Virus = A coronavirus that infects liver cells of mice leading to severe hepatitis
30
Different strains of mice exhibit ________________ to MHV
Different strains of mice exhibit **Different susceptibility** to MHV
31
What mice strains are/are NOT susceptible to MHV?
**BALB/c** = SUSCEPTIBLE to MHV
**SJL/J**= NOT Susceptible to MHV!
32
BALB/c
A strain of mouse that IS susceptible to MHV
33
SJL/J
A strain of mouse that IS NOT susceptible to MHV
34
When analyzing the receptors of the 2 mice strains (BALB/c + SJL/J) what was found? (results)
Observed that the receptors Bgp1a and Bgp1b LOOKED similar but had DIFFERENT AA sequences!
35
What are the names of the MHV receptors for BALB/c and SJL/J mice?
BALB/c == Receptor name is **Bgp1a**
SJL/J == Receptor name is **Bgp1b
36
Why do BALB/c and SJL/J mice differ in their susceptibility to MHV?
Because of differences in their receptors (that MHV binds to)
BALB/c == Bgp1a == HIGH MHV binding affinity
SJL/J == Bgp1b == LOW MHV binding affinity
--> The difference in binding affinity is most likely due to these different AA sequences
37
What was the overall conclusion of the study investigating MHV susceptibility?
The viral attachment protein:host receptor interaction determines susceptibility of mice to MHV
38
What is the main attachment protein on influenza viruses?
HA = hemagglutinin
39
What does HA bind to?
**Sialic Acid (SA)** BUT only when it is the TERMINAL RESIDUE on a glycoprotein
40
Different influenza strains infect different animals, what determines which animal a strain will infect?
Which linked form of sialic acid the strain preferentially binds to!
41
In birds, what form of SA do Influenza strains preferentially bind to?
**ALPHA-2,3** linked sialic acid
42
In humans, what form of SA do Influenza strains preferentially bind to?
**ALPHA-2,6** linked sialic acid
43
What is meant by the "forms" of sialic acid? (specifically in the context of influenza binding)
SA can exist in different linked states depending on what carbon/how it connects to the polysaccharide chain of the glycoprotein it's attached to
44
Why do bird strains and human strains of Influenza not infect each other (mostly)?
Because the two strains preferrentially bind to different forms of linked sialic acid!
--> Birds and humans have SA linked to glycoproteins via 2 different glycosidic linkages
45
What mainly determines host range?
Viral attachment protein : Receptor binding event
46
As infection requires binding of a virus to a host cell, what is a potential source for viral treatments? Why?
We can prevent infection by PREVENTING viral attachment/binding in the first place == potential source for treatment
--> Prevents the virus from attaching which is a necessary step for entry/infection to occur; no attachment, no infection
47
What is Maraviroc?
An anti-viral drug that blocks the co-receptors on the host cells of HIV, preventing its attachment
--> Drug designed to block viral attachment by preventing the HIV viral attachment proteins from interacting with the host receptors
48
After a virus has sucdessfully bound to a host cell, what must occur next?
What is the problem with carrying out this next step?
The virus must GAIN ENTRY!
49
What is a main barrier to viruses gaining entry into host cells?
The host cell PLASMA MEMBRANE (+ cell wall/other outer layers)
--> Viral particles are too big to pass through it!
50
After an enveloped virus binds to a host cell, what elements separate the viral genome from the cell interior?
TWO MEMBRANES (lipid bilayers)
1) Host plasma membrane
2) Viral envelope
51
What are the two methods enveloped viruses use to gain entry?
1) **Direct fusion** with host PM
2) **Endocytosis 1st method** (followed by fusion with the endosomal membrane)
52
No matter the method, ALL enveloped viruses must do what in order to get their genome into the cell environment?
Undergo a FUSION event (of barrier membranes)
--> ALL enveloped viruses conduct a fusion event between the viral envelope and some host cell membrane
== allows the release of the viral genome
53
What differs between the two methods of entry of enveloped viruses?
WHEN the fusion event occurs in their processes
54
What is the fusion strategy of HIV?
Direct fusion with PM host cell
55
What is the fusion strategy of Influenza?
Endocytosis 1st followed by fusion event with endosomal membrane
56
What is the host cell receptor HIV binds to?
CD4
57
What are the viral attachment proteins found on HIV?
gp41 = The "stick" part, embedded in the viral envelope
gp120 = The terminal "ball" part, attached to gp41
58
Fusion Peptide
A short string of hydrophobic AAs that mediates the merging of membranes between virus and host cell
59
What is the process of HIV entry?
1) gp120 binds to host cell CD4
2) Binding triggers conformational change of the gp120-CD4 complex
3) Conformational change causes gp120 to also bind to the host cell **co-receptor**
4) Binding to co-receptor triggers a conformational change in gp41
5) Conformational change of gp41 reveals its FUSION PEPTIDE (domain)
5) The exposed fusion peptide interacts with the plasma membrane
6) The fusion peptide-PM interaction PULLS gp41 with it, close to the PM and gp41 "pulls" the virion with it and sets it close to the PM
7) The viral envelope and PM in close apposition interact and the fusion event occurs
8) Viral nucleocapsid is released into the cell!
60
What is the role of the coreceptor in HIV entry?
"Holds" gp120 in such a way that gp41 is forced to configure itself in a way in which its fusion peptide is exposed!
61
Together, gp41 and the host coreceptor do what?
Facilitate fusion!
(between HIV envelope and host cell membrane)
62
What was the first anti-retroviral drug?
Enfuvirtide
(Brand name = Fuzeon)
63
What does Enfuvirtide (Fuzeon) bind to?
How does this drug work?
Fuzeon binds to **gp41**
--> Immobilizes gp41 so that when the fusion peptide interacts with the PM, gp41 stays in place
== viral envelope is NOT pulled to the plasma membrane
== NO FUSION == NO ENTRY! --> No infection
64
What principle characteristic of viral infection process does Enfuvirtide (Fuzeon) target?
Viral entry!
(specifically via fusion with the host cell PM)
65
When influenza HA binds to host cell terminal SA (off of glycoprotein), what happens?
Receptor-mediated endocytosis is triggered!
66
What is the process of Influenza entry?
1) HA (influenza attachment protein) binds to SA (on glycoprotein receptor of host)
2) HA-SA binding triggers the initiation of receptor-mediated endocytosis
3) The ENTIRE influenza virion is engulfed within an endosome
4) The endosome becomes acidified (through normal cell processes)
5) The drop in pH triggers a conformational change in HA
6) HA conformational change EXPOSES fusion peptide!
7) The fusion peptide interacts with the endosomal membrane
8) The viral envelope and endosomal membrane are brought together by this interaction
9) Fusion of the membranes results and the nucleocapsid is released from the virion + endosome into the cytoplasm
67
Endocytosis
Generalized process for the uptake of external materials by cell membrane extension or invagination/fusion to form an endosome in the cytoplasm
68
Most endocytic viruses are described as....
pH dependent!
--> Because the viruses rely on low pH in order to escape the endosome
69
What is a main difference between enveloped and non-enveloped viral entry?
Enveloped = Requires a FUSION event!
Non-enveloped = does NOT require a fusion event!
70
Non-enveloped viral entry does NOT require ________ but instead requires ____________________ of the host cell membrane
Non-enveloped viral entry does NOT require **membrane fusion** but instead requires **direct penetration** of the host cell membrane
71
What is a similarity between enveloped and non-enveloped viral entry?
BOTH can use receptor-mediated endocytosis?
72
What is the most common method of non-enveloped viral entry?
Receptor-mediated endocytosis
73
General endocytosis process for non-enveloped viruses:
1) Viral particle binds to host cell receptor
2) Binding event triggers endocytosis to initiate
3) Endocytosis results in the internalization of the viral particle within an endosomal vesicle
4) Acidification of the endosome triggers conformational change within the proteins of the VIRAL CAPSID
5) New conformation of capsid protein reveals a PORE-FORMING DOMAIN
6) A hole is created within the endosome membrane through which the viral nucleocapsid or genome exits into the cytoplasm
74
Explain the differences between endocytosis process for enveloped and non-enveloped viruses:
1) **Engulfment = same for both**
2) **Acidification of the Endosome = DIFFERENT**
--> Enveloped = low pH causes conformational change to expose the fusion peptide in the viral attachment protein
--> Non-enveloped = low pH causes conformation change to expose a pore forming domain in the capsid protein/s
3) **Endosomal Escape = DIFFERENT**
--> Enveloped = fusion peptide exposure causes fusion of the endosome with the viral envelope, releasing the nucleocapsid
--> Non-enveloped = pore-forming domain exposure causes hole/s to form in the endosome, the nucleocapsid or viral genome escapes through these holes!
75
In endocytosis process, what does the acidification of the endosome do to an engulfed non-enveloped virion?
It causes conformational changes in the capsid proteins of the virion == triggering the exposure of a pore forming domain!
(that will allow the virion to make a hole in the endosome to then escape)
76
During bacteriophage entry, what actually enters the host cell?
ONLY the bacteriophage GENOME enters!
--> The other components of the phage never enter the host cell!
77
After phage particles interact with the host cell receptor, how do they get their genomes inside the host?
By injecting their GENOME through the cell wall and cell membrane of their host bacteria!
78
What is the overall process of bacteriophage entry into a host cell?
1) Phage tail fibers facilitate attachment of the phage to the host cell!
2) Virus **injects** the genome into the cell cytoplasm
3) Empty viral capsid may or may not detach from cell
79
Within the T4 bacteriophage, interaction between the phage and its host cell receptors causes what?
CONTRACTION of the T4 tail! (eventually leads to genome injection)
80
Pilot Protein
A protein that plays a crucial role in facilitating the transfer or delivery of other molecules, especially DNA or RNA, from one place to another
--> acts as a guide or leader, ensuring the efficient movement and interaction of these molecules
81
What is the process of entry for the T4 phage?
1) Tail fibers bind to host cell receptors (attachment)
2) Binding triggers contraction of the T4 tail!
3) The tail CORE pushes through the cell wall
4) A pilot protein binds to the page DNA
5) Pilot-bound phage DNA is able to penetrate the plasma membrane
== cell injected with viral genome!
82
What is the role of the pilot protein in T4 phage entry?
To direct the phage DNA to the PM to allow it to penetrate the PM (and therefore enter the cytoplasm)
83
What do plant viruses rely upon to gain entry into host cells?
What is an example?
OUTSIDE FORCES!
--> Forces that provide a region of entry (area of damage that produces an opening that would allow the virus to enter!)
--> Ex: Insects feeding on plants (damages plant cell wall and cuticle providing an opening for viral infection)
-->
84
How do plant viruses spread cell to cell within the same plant?
Via cytoplasmic connections between cells (plasmodesmata)
85
How are plant viruses spread from plant to plant?
Usually through some external method of transmission
(Ex: insect eats from an infected plant and then goes and feeds on an uninfected plant = can bring the infection to the uninfected one!)
86
After a virus has entered a cell, what must occur before replication can begin?
Uncoating!
87
Uncoating
The process where a virus makes its genetic material ACCESSIBLE to host cell machinery!
= disassembly of the protective protein shell (capsid)
88
Is uncoating needed for all viruses?
If not, which ones need it?
**NO**: not all viruses must undergo an uncoating step in the host cell!
--> Uncoating is only needed for those viruses that ENTER the host cell with MORE than just the viral genome!
89
Do bacteriophages have to uncoat?
NO: bacteriophages do not need to uncoat as they inject only their genomes into the host cell
== genetic material is already accessible upon viral entry!
90
What are the 2 most common methods of uncoating?
1) Conformational change in capsid proteins
2) Proteolytic processing of capsid proteins
91
What is the process of uncoating for Poliovirus?
1) Poliovirus (160S form) CAPSID binds to the poliovirus receptor on host cell
2) Binding triggers irreversible conformation change in the poliovirus capsid
3) The conformational change causes the VP4 internal capsid protein to be exposed and degraded!
== Altered poliovirus virion is created! (135S form)
4) Loss of VP4 exposes VP1 in the poliovirus capsid
5) VP1 extends down and forms a CHANNEL through the host plasma membrane
6) Viral RNA is extruded from the capsid through the VP1 channel
92
What are the 2 forms of the poliovirus during entry/uncoating?
**160S** = Native form (VP4 intact, VP1 in place)
**(Capsid INTACT)**
**135S** = Altered form (VP4 lost, VP1 exposed)
**(Partially disassembled capsid)**
93
What are VP4 and VP1?
VP4 and VP1 are BOTH **poliovirus capsid proteins**!
VP4 = An INTERNAL capsid protein
VP1 = An EXTERNAL capsid protein (forms part of the outer shell)
94
PVR
Poliovirus Receptor
--> a host cell receptor that binds to poliovirus
