Ch.15, Part 2 - Cell Compartments Flashcards Preview

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Flashcards in Ch.15, Part 2 - Cell Compartments Deck (78)
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1

Provide a general overview of the major outward secretory pathway.

Major outward secretory pathway - starts w synth of proteins on ER mem → entry into ER → thru Golgi → cell surface OR side branch leading thru endosomes to lysosomes.

2

Provide a general overview of the major inward endocytic pathway.

Major inward endocytic pathway - responsible for ingestion and degradation of EC molecules; moves materials fr pmem → thru endosomes → lysosomes.

3

Ea xprt vesicle must take w it only the proteins approp to its destination and must fuse only w approp target mem, thereby maintaining the distinct protein/lipid composition of organelles. What feature of xprt vesicles enables this?

Proteins displayed on surface of xprt vesicles ensures proper vesicle cargo/destination → maintains distinct protein/lipid comp of organelle.

4

Vesicle budding is driven by the assembly of a ________.

Vesicle budding is driven by the assembly of a protein coat.

  • Coated vesicles - vesicles w distinctive protein coat on cytosolic surface; bud fr mems.
    • Bud fr parent organelle → vesicle sheds coat → allows vesicle mem to interact directly w target mem.
    • Several types of coated vesicles, ea w distinctive protein coat.
    • Coat helps shape mem into a bud and captures molecules for onward xprt.

5

Coated vesicles are those w distinctive protein coat on their _______ (cytosolic/non) surface → bud fr parent organelle → vesicle sheds coat → allows vesicle to ______________________.

Coated vesicles are those w distinctive protein coat on their cytosolic surface → bud fr parent organelle → vesicle sheds coat → allows vesicle to interact directly w target mem.

  • Coat helps shape mem into a bud and captures molecules for onward xprt.

6

________-coated vesicles are v well studied and bud fr both Golgi on outward secretory pathway and fr pmem on inward endocytic pathway.

Clathrin-coated vesicles - well-studied; bud fr both Golgi on outward secretory pathway and fr pmem on inward endocytic pathway.

7

Describe the endocytic mechanism via clathrin-coated vesicles.

Endocytic mechanism:

  • At pmem, ea vesicle starts off as a clathrin-coated pit → clathrin molecules assemble into a basket-like network on cytosolic surface of mem → assembly process starts shaping mem into a vesicle.
  • A small GTP-binding protein (dynamin) assembles as a ring around neck of ea deeply invaginated coated pit → dynamin and assembly proteins causes ring to constrict → pinch off vesicle fr parent mem.
  • Other xprt vescles (w diff protein coats) form in similar way.

8

________ are a second class of coat proteins that secure the clathrin coat to the xprt vesicle mem and help select cargo molecules for xprt.

Adaptins are a second class of coat proteins that secure the clathrin coat to the xprt vesicle mem and help select cargo molecules for xprt.

  • Diff types of adaptins for binding diff types of cargo receptors in diff organelles.

9

T/F: The protein coat itself plays no part in choosing specific molecules for xprt.

True

The protein coat itself plays no part in choosing specific molecules for xprt.

  • Adaptins - a second class of coat proteins; secure clathrin coat to vesicle mem and help select cargo molecules for xprt.

10

Molecules for onward xprt (beyond ER) carry specific xprt signals that are recog by __________ in Golgi/pmem → _______ help capture specific cargo molecules by trapping the _________ that bind them.

Molecules for onward xprt (beyond ER) carry specific xprt signals that are recog by cargo receptors in Golgi/pmem → adaptins help capture specific cargo molecules by trapping the cargo receptors that bind them.

  • Thus, a selected set of cargo molecules, bound to their specific receptors, is incorporated into the lumen of ea newly formed clathrin-coated vesicle.

11

_____-coated vesicles are another class of coated vesicles; involved in xprt b/w ER and Golgi, and fr one part of Golgi to another.

COP-coated vesicles (COP = "coat protein") - another class of coated vesicles; involved in xprt b/w ER and Golgi, and fr one part of Golgi to another.

12

After budding fr a mem, vesicles are often actively xprtd by ________ that move along cytoskeletal fibers

After budding fr a mem, vesicles are often actively xprtd by motor proteins that move along cytoskeletal fibers

13

Vescile docking deps on ______ and ______.

Vescile docking deps on tethers and SNAREs.

14

Vesicles identify, bind, and fuse w target organelle via specific surface markers on both the vesicle surface and target mem, incl pmem. What types of proteins are involved in this process?

  • Rab proteins - diverse family of monomeric GTPases; specific to ea type of vesicle.
  • Tethering proteins - filamentous protein on cytosolic surface of target mem; recog/bind specific Rab proteins.
  • SNAREs - family of xmem proteins; involved in vesicle recog, ensure proper docking, help catalyze fusion and deliver soluble contents into organelle, and add vesicle mem to target mem.
    • v-SNAREs = SNAREs on vesicles
    • t-SNAREs = SNAREs on target mem

15

Rab proteins, tethering proteins, and SNAREs help direct xprt vesicles to their target mems. Describe the tethering, docking, and fusion processes.

  • Tethering - tethering protein on target mem recogs/binds vesicles bearing approp Rab protein → tethering protein guides vesicle toward target mem.
  • Docking - v-SNAREs binds complementary t-SNAREs → firmly docks vesicle in place.
    • Docking reqs only that the two mems come close enough for SNAREs to interact.
  • Fusion - v- and t-SNAREs catalyze final fusion of vesicle and target mems.
  • Post-fusion - SNAREs unwind → reused.

16

Rab proteins, tethering proteins, and SNAREs help direct xprt vesicles to their target mems. After tethering and docking, describe why the two mems don't spontaneously fuse t/g.

Fusion - v-/t-SNAREs catalyze final fusion of vesicle and target mems. Sometimes reqs special stim signal:

  • Fusion reqs much closer approach: bilayers must come w/i 1.5 nm of ea/o so that their lipids intermix → water must be displaced fr hphilic surfaces of mems (highly energ unfav) → prevents mems fr fusing randomly.
  • Thus, all mem fusions must be catalyzed by specialized proteins (SNAREs): SNAREs form fusion complex (wrap around ea/o) → winding acts like a winch, pulling bilayers in close proximity and squeezing out any remaining water → enables random/spont fusion of mems.

17

The budding of clathrin-coated vesicles fr euk pmem fragments can be observed when adaptins, clathrin, and dynamin-GTP are added to the mem preparation. What would you observe if you omitted adaptins?

Clathrin coats cannot assemble in absence of adaptins that link clathrin to mem. At high clathrin concens and under approp ionic conditions, clathrin cages assemble in solution, but they are empty shells, lacking other proteins, and they contain no mem.

  • This shows that the info to form clathrin baskets is contained in teh clathrin molecules themselves → can self-assemble.

18

The budding of clathrin-coated vesicles fr euk pmem fragments can be observed when adaptins, clathrin, and dynamin-GTP are added to the mem preparation. What would you observe if you omitted clathrin or dynamin?

  • W/o clathrin - adaptins still bind to receptors in mem, but no clathrin coat can form and thus no clathrin-coated pits or vesicles are produced.
  • W/o dyanmin - Deeply invaginated clathrin-coated pits form on the mem, but they do not pinch off to form closed vesicles.

19

The budding of clathrin-coated vesicles fr euk pmem fragments can be observed when adaptins, clathrin, and dynamin-GTP are added to the mem preparation. What would you observe if the pmem fragments were fr a prok cell instead?

Prok cells do not perform endocytosis → does not contain any receptors w approp cytosolic tails that could mediate adaptin binding → no clathrin can bind and no clathrin coats can assemble.

20

Most proteins are covalently modified in the ________.

Most proteins are covalently modified in the ER.

21

T/F: most proteins that enter ER are chemically modified there.

True

Most proteins that enter ER are chemically modified there.

  • E.g. disulfide bonds, glycosylation.

22

Most proteins that enter ER are chemically modified there. One such mod is disulfide bonds. Describe how these bonds form and what purpose they serve.

Disulfide bonds - enzyme in ER lumen catalyzes oxidation of pairs of cysteine side chains → help stabilize struc of proteins that will encounter degradative (oxidative) enzymes and changes in pH outside cell (e.g. both excreted proteins and those in EC side of pmem).

  • Recall: disulfide bonds do not form in cytosol bc more reducing agents and stable pH.

23

Glycosylating enzymes in the ________ catalyze the _________ (covalent/non) attachment of short, branched _________ side chains to proteins in ER lumen/mem → converted to _________.

Glycosylating enzymes in the ER catalyze the covalent  attachment of short, branched oligosaccharide side chains to proteins in ER lumen/mem → converted to glycoproteins.

24

Glycosylating enzymes in the ER catalyze the covalent  attachment of short, branched oligosaccharide side chains to proteins in ER lumen/mem → converted to glycoproteins. What functions do glycoproteins serve?

Various functions:

  • protect protein fr degradation
  • hold protein ER until properly folded
  • help guide protein to target organelle by serving as xprt signal for packaging protein into approp xprt vesicles
  • for EC-side mem proteins, oligos contrib to glycocalyx (carb layer) and help cell-cell recog.

25

T/F: Most cytosolic proteins are glycosylated.

False

V few cytosolic proteins are glycosylated; if they are, only single sugar attached.

  • Recall: glycosylating enzymes in ER—not cytosol—catalyze covalent attachment of short branched oligo side chains to proteins in ER lumen/mem → converted to glycoproteins.

26

Where are the oligosaccharides located prior to attachment during glycosylation?

Oligos are originally attached to specialized lipid (dolichol) in ER mem.

27

T/F: During glycosylation, oligos are attached one-by-one to the approp glycosylation site.

False

Oligos are attached en bloc (as preformed branch of 14 sugars) to proteins w approp glycosylation site immediately as it emerges into ER lumen during protein xlctn.

28

The glycosylation site is a simple seq of ___ (#) AAs, one of wh is ________.

The glycosylation site is a simple seq of three AAs, one of wh is asparagine (asn, N).

  • I.e. all asn's are not glycosylated, must be part of partic tripeptide seq: either asn-X-Ser or asn-X-thr; X is almost any AA; Serine = ser, S; Threonine = thr, T.

29

Glycosylation is catalyzed by a _______ (cytosolic/mem-bound/lumenal) enzyme in ______ (mult/one) enzymatic step.

Glycosylation is catalyzed by mem-bound enzyme in a single enzymatic step.

  • Oligosaccharyl transferase has active site exposed on lumenal side of ER mem, wh explains why cytosolic proteins are not glycosylated in this way.
  • N-linked oligo side chains - those linked to asparagine (asn, N) NH2 group; by far most common type.

30

__-linked oligo side chains are by far the most common type of glycoprotein.

N-linked oligo side chains are by far the most common type of glycoprotein.

  • N-linked oligo side chains - those linked to asparagine (asn, N) NH2 group.