4. Virus Entry

Question 1

is virus entry passive or active? why?

Answer 1

ACTIVE –> virus particles are too big to diffuse across the plasma membrane so they must be taken up by specific pathways

Question 2

describe virus particles finding the ‘right’ cell (4 steps)

Answer 2

1. adhere to cell surface via ELECTROSTATICS –> no specificity
2. attach to specific receptor via glycosylated protein
3. penetration
4. transport and uncoating to release genome

Question 3

describe receptors/attachment factors on the outside of the cell

Answer 3

proteins can be indirectly anchored by tethering to transmembrane proteins and some transmembrane proteins are glycosylated

Question 4

describe what receptors/attachment factors look like on the inside of the cell

Answer 4

proteins can be indirectly anchored by tethering to transmembrane proteins and there are some lipid anchored proteins that don’t span the membrane

Question 5

what specific type of molecule do viruses bind when they bind cell receptors? what is most common?

Answer 5

sugars –> sialic acid

Question 6

what are attachment factors?

Answer 6

cell surface components involved in virion BINDING

Question 7

what are receptors?

Answer 7

cell surface components involved in virion binding AND entry

Question 8

what are some ways that receptors can trigger virion entry? (3)

Answer 8

1. conformational changes
2. cell signaling
3. mediate endocytosis

Question 9

are attachment factors and receptors only present in cellular membranes to allow virus binding?

Answer 9

NO!! they have specific cellular function

Question 10

what is an example of an attachment factor?

Answer 10

sialic acid

Question 11

viral receptors allow _______

Answer 11

viral receptors allow tropism

Question 12

what are the 4 possibilities of viruses matching a receptor?

Answer 12

1. different viruses can have the same receptor
2. viruses in the same family may bind different receptors
3. one virus may bind multiple receptors
4. a virus may use different receptors on different cell types

Question 13

how do naked viruses bind their receptors? (2 ways)

Answer 13

1. capsid surface
2. protrusions (spike proteins)

Question 14

what do naked viruses do once they bind receptors? (2 possibilities)

Answer 14

1. inject their genomes
2. trigger endocytosis

Question 15

how do enveloped viruses bind their receptors? an example?

Answer 15

using transmembrane glycoproteins (ex. sialic acid)

Question 16

what are the 2 main virus entry pathways?

Answer 16

1. endocytosis
2. membrane fusion

Question 17

which type of virus entry pathways do naked viruses use?

Answer 17

endocytosis only

Question 18

which type of virus entry pathways do enveloped viruses use?

Answer 18

endocytosis and membrane fusion

Question 19

2 examples of viruses that use endocytosis

Answer 19

1. adenovirus
2. poliovirus

Question 20

2 examples of viruses that use membrane fusion

Answer 20

1. HIV (at plasma membrane)
2. influenza (inside endosomes)

Question 21

what occurs during membrane fusion? what stimulates this process

Answer 21

2 membranes come together to become 1

stimulated by fusion proteins encoded by enveloped viruses

Question 22

what are the 4 steps for viral fusion at the plasma membrane for HIV?

Answer 22

1. gp120 trimer binds to CD4 receptor and undergoes conformational change to allow binding to co-receptor CCR5
2. gp41 (fusion peptide) inserts into cell membrane to form pre-hairpin intermediate
3. gp41 HR2 folds back onto HR1 to form the six-helix bundle that brings the 2 membranes together
4. a hemifusion stalk forms and becomes fusion pore

Question 23

what type of protein is gp41?

Answer 23

fusion protein encoded by virus

Question 24

how can gp41 insert into the cell membrane?

Answer 24

gp120 undergoes conformational changes so it binds to co-receptor which allows the hydrophobic part of gp41 to be exposed and it can then go to the cell membrane

Question 25

what type of secondary structure do HR2 and HR1 have?

Answer 25

alpha helix

Question 26

what changes in an endosome to trigger the fusion of some enveloped viruses?

Answer 26

low pH in the endosome

Question 27

what is the first part of viral entry for influenza?

Answer 27

endocytosis

Question 28

describe influenza entry in an endosome (4 steps)

Answer 28

1. hemagglutinin protein (HA) binds to cell surface receptors that have sialic acid 2. HA-sialic acid interactions promote viral endocytosis 3. endosome acidifies 4. acidification triggers a conformational change in HA that leads to membrane fusion

Question 29

what are the 6 steps of influenza membrane fusion?

Answer 29

1. upon acidification, HA (fusion protein) is exposed and directed towards the target membrane by extending the coiled coil 2. fusion proteins insert into the target membrane links the viral and target membranes 3. if there are enough fusion proteins, a fusion pore can form 4. a six-helix bundle forms 5. hemifusion occurs as membranes are brought close together 6. fusion occurs with the completion of the six-helix bundle

Question 30

how are class I fusion proteins aligned in the membrane?

Answer 30

perpendicular

Question 31

what type of secondary structure are class I fusion proteins made of?

Answer 31

mostly alpha-helix

Question 32

do class I fusion proteins form trimers or dimers?

Answer 32

trimers

Question 33

how are class II fusion proteins aligned in the membrane?

Answer 33

parallel to the membrane

Question 34

what type of secondary structure are class II fusion proteins made of?

Answer 34

mostly beta sheets

Question 35

do class II fusion proteins form trimers or dimers?

Answer 35

dimers

Question 36

why are class II fusion proteins hidden until they are needed? when do they stop 'hiding'?

Answer 36

class II fusion proteins are hydrophobic so must be hidden until the right time bc don't want it to attract the membrane --> conformational changes due to endosome acidification allow the fusion protein to stick into the cell membrane

Question 37

do class I and class II fusion proteins have similar mechanisms?

Answer 37

yes, even though the proteins are different

Question 38

what is the general mechanism for class I and II fusion proteins? 3 steps

Answer 38

1. fusion protein hidden in native conformation 2. conformational change allows insertion of fusion protein into the membrane 3. protein/hairpin folds back to bring the membranes close together and initiate fusion

Question 39

describe membrane fusion of Dengue virus in endosomes (5 steps)

Answer 39

1. dimer of E protein is anchored in the viral membrane 2. upon acidification of endosome, the dimer dissociates and inserts the fusion loop into the endosomal membrane 3. E protein trimerizes, allowing domain III to bind to a core trimer 4. domain III flips to the side of the trimer, pulling the stem and transmembrane segments toward the endosomal membrane 5. transmembrane segment is brought to the fusion loop, allowing hemi-fusion and opening of a fusion pore

Question 40

can naked viruses mediate membrane fusion?

Answer 40

NO!!!

Question 41

how do non-enveloped viruses get through cell membranes? ex. adenovirus and ex. poliovirus

Answer 41

non-enveloped viruses rely on the rupture of cell membranes adenovirus --> ruptures endosome poliovirus --> creates membrane channel/pore

Question 42

how do viral particles move within cells?

Answer 42

must be transported by vesicles or actin and microtubule cytoskeletons --> too big to move via diffusion

Question 43

when can viruses enter the nucleus? (2)

Answer 43

1. during cell division when the nuclear membrane is broken down 2. thru nuclear pore complex

Question 44

what is the nuclear pore complex?

Answer 44

1000s of nuclear localization signals that dock at the nuclear pore along the nuclear membrane

Question 45

how does the influenza virus get into the nucleus?

Answer 45

genome is segmented and segments diffuse thru the nuclear pore

Question 46

how do HSV-1 and adenovirus get into the nucleus?

Answer 46

virus docks at the nuclear pore and injects the viral genome into the nucleus

Question 47

how does parvovirus get into the nucleus?

Answer 47

disrupts the nuclear membrane and makes its own pore