Chapter 12: Protein Sorting & Transport Flashcards
what are the components of the secretory pathway?
-RER
-SER
-Golgi
-lysosomes
what is the function of the RER
protein:
- folding
- modification
- processing
what is the function of the smooth ER
membrane & lipid synthesis
what is the function of the golgi
protein processing and sorting
the secretory pathway is connected by
vesicular transport
what experiments defines the secretory pathway?
labeling new proteins w/ radioactive amino acids
proteins synthesized from free ribosomes in the cytosol can be transported to
posttranslational import
- nucleus
- mitochondria
- chloroplasts
- peroxisomes
proteins synthesized from membrane-bound ribosomes can be transported to
cotranslational import
- Golgi
- endosomes
- plasma mem.
- lysosomes
- peroxisome membrane
- secretory vesicles
- nuclear mem.
protein sorting begins during
translation
which types of proteins retain their signal sequence?
cytosolic
proteins translated into _________ have their signal sequence cleaved
microsomes
signal sequences are removed by
proteolytic cleavage
what are the 2 mechanisms for targeting proteins to the ER
- cotranslational translocation
- posttranslational translocation
secreted proteins are AKA
lumenal proteins
are secreted proteins inserted into the ER membrane?
no they are released into the lumen of the ER
membrane proteins are…instead of…
inserted into the ER membrane instead of being released into the lumen
how are integral membrane proteins embedded into the ER mem.
by hydrophobic regions that span the phospholipid bilayer
the hydrophobic regions that embed integral membrane proteins are also known as
STS
stop-transfer-sequence
the STS are typically
α-helical
what are the 3 signal sequences
- ERSS
- STS
- ISS
the ERSS stands for
ER signal sequence
the STS stands for
stop transfer sequence
the ISS stands for
internal start sequence
list the characteristics of a type I protein
- ERSS @ N-terminus
- contains signal peptidase cleavage site
- STS
- N-terminus at ER lumen
- C-terminus at cytoplasm
list the characteristics of a type II protein
- internal ERSS
- no signal peptidase cleavage site
- N-terminus at cytoplasm
list the characteristics of a type III protein
- internal ERSS
- no signal peptidase cleavage site
- N-terminus at ER lumen
the internal ERSS of a type II protein acts as a
start transfer sequence
the internal ERSS of a type III proteins acts as a
stop transfer sequence
what happens to proteins that contain a ERSS at the c-terminus?
they’re recognized by TRC40 which takes it to the GET1-GET2 receptor
the positive inside rule states that
the more positive side will be found in the cytoplasm
lumens of ER and Golgi are topologically equivalent to
the exterior of the cell
list the 5 primary roles of ER lumenal proteins
- folding of newly translated polypeptides
- formation of disulfide bonds
- assembly of multi-subunit proteins
- N-linked glycosylation
- addition of GPI anchors
chaperones are members of what protein family
Hsp70
what specific chaperon is used in protein folding
BiP
BiP binds to
hydrophobic regions
BiP uses _______ to assist in folding. what does this molecule do?
ATP
- opens & closes BiP binding sites
what indicates misfolding in a protein
too many exposed hydrophobic regions
misfolded protein is targeted back to the cytosol through a
ubiquitin ligase complex
mechanism for protein breakdown
protein is ubiquitylated at the cytosolic side and degraded by a proteasome
what is required for protein degradation by a proteasome?
amino acids
describe N-linked glycosylation
glycosylation of the nitrogen of Asn within the consensus sequence
what is the consensus sequence for N-glycosylation
N-X-S/T
where does N-linked glycosylation occur
within the ER
what are glycosylphosphatidylinositol (GPI) anchors
glycolipids containing phosphatidylinositol
GPI anchors are assembled in the
ER membrane
GPI anchors are added to the __________ of polypeptides
carboxy terminus
GPI anchors are ultimately exposed to
the outside of the cell
list 4 other types of ER protein modifications
- chemical mod. of individual amino acids
- proteolytic cleavage
- binding to prosthetic groups
- formation of multisubunit proteins or molecular complexes
the smooth ER is a major site for
lipid synthesis
lipid synthesis occurs on the
cytosolic side of the membrane
what are the 3 main types of lipids
- phospholipids
- glycolipids
- cholesterol
phospholipids are synthesized from
water soluble precursors
what is the function of flippases
catalyze the translocation of phospholipids from the OM to the IM
transport from the ER to the Golgi is known as what type of transport?
anterograde
transport from the Golgi to the ER is known as what type of transport?
retrograde
the Golgi contains NO
functional lumenal proteins
define cisternae
flattened membrane enclosed stacks
what are the 3 components to the Golgi
- cis
- medial
- trans
the cis face is responsible for
- being port entry site
- protein folding & modifications
the cis face is always oriented towards the
nucleus
the medial Golgi is responsible for
most metabolic activities
the trans Golgi is responsible for
- being exit port
- more modification
- directing cellular traffic
how does the trans face of the Golgi direct cellular traffic
by acting as the protein sorting and distribution center
the Golgi complex is a factory for the
processing and sorting transport proteins which have been received from the ER
ALL proteins that enter the secretory pathway enter the _________ regardless of their final subcellular destination
Golgi
what is the c-terminus sequence that targets proteins to be retained in the ER
KDEL OR KKXX
how do ER proteins return to the ER
via a recycling pathway after their transport & modification in the Golgi
what are the 2 theories to protein movement once they’ve entered the Golgi
- ERGIC gets reformed
- rebuilding cis face
- degrading trans face - stacks remain stationary
- proteins move through vesicular transport
what 3 things occur in the Golgi apparatus
- protein processing & modification
- lipid metabolism
- protein sorting
what are the 3 types of protein glycosylation that occurs in the Golgi
- modification of N-linked oligosaccharides
- lysosomal protein modification
- o-linked glycosylation
in modification of N-linked oligosaccharides what is the role of glycosyltransferases
adding sugars
in modification of N-linked oligosaccharides what is the role of glycosidases
remove the sugars
lysosomal protein modification is the
addition of mannose-6-phosphate
where is mannose-6-phosphate added to
the N-linked oligosaccharide of a lysosomal protein
what is a signal patch
a particular 3-D conformation that targets glycoproteins to lysosomes
mannose phosphorylation can ONLY target glycoproteins to lysosomes if they have a
signal patch
o-linked glycosylation is the addition of a
carbohydrate to the O of a serine or threonine
lipid metabolism is the
synthesis of glycolipids and sphingomyelin
glycolipids and sphingomyelin are synthesized from
ceramides
what is the only non-glycerol phospholipid in membranes
sphingomyelins
sphingomyelins are formed on the
lumenal side of the mem.
_____________ cannot be flipped. why?
glycolipids
- very hydrophobic
glycolipids act as
important cell surface markers and signaling molecules
glycolipids on only found on what side of the Golgi bilayer
lumenal
after vesicular transport both sphingomyelin and glycolipids are localized to the
exterior half of the plasma mem.
processing of proteoglycans refers to the
addition of carbohydrate chains and sulfate groups to a polypeptide
the proteoglycan chains are composed for what 2 repeating disaccharides
- chondroitin
- keratin
what are the 3 potential destinations from the trans Golgi
- lysosomes
- cell mem.
- secretion
secreted proteins are considered to be constitutive if they are
secreted as soon as they’re made and processed
give examples of constitutive secreted proteins
- cytokines from immune cells
- albumin from the liver
- adhesion proteins
secreted proteins are considered to be regulated if they are
secreted out of the cell upon receiving signal from the exterior of the cell
give examples of regulated secreted proteins
- neurotransmitters
- hormones
- digestive enzymes
in polar cells, like epithelial cells, what must be regulated
direction of transport & secretion
ALL functional proteins retained within the Golgi are associated w/
the Golgi mem.
what are the 3 characteristics of Golgi-localized proteins
- short N-terminus facing cytosol
- single transmembrane a-helix
- large C-terminal domain facing Golgi lumen containing catalytic site
what increases progressively from the ER to the plasma
the thickness of cellular mem.
what are the 2 hypothesis as to what keeps the Golgi proteins associated with the Golgi?
- membrane proteins move from compartment to compartment until the membrane length exceeds fatty acid tails
- Golgi enzymes bind to cytoskeletal proteins preventing their diffusion into transport vesicles
define coatamers
protein complex that coats membrane bound transport vesicles
what are the 3 types of coatamers?
-COPI
-COPII
-clathrin
COPI is responsible for what type of transport
retrograde
COPII is responsible for what type of transport
anterograde
define budding
the formation of a transport vesicle at the surface of a membrane-bound compartment
define fusion
the incorporation of a transport vesicle into a membrane-bound component
coat proteins are
trimers
upon polymerization the trimers forms
numerous hexagons
complete binding of trimers creates
a bent surface with convex and concave sides
the growing complex of trimers provides
the mechanical force to pull the mem. into a bud
as coat proteins assemble around a budding vesicle what is selected?
adaptor molecules
how is the appropriate cargo incorporated into the vesicle?
mem. receptors bind to their respective cargo ligand
what is the function of dynamin
to pinch off the vesicle during budding
what dissociates from the transport vesicle once it enroute to final destination
clathrin
clathrin coated vesicles can travel
both anterograde and retrograde
clathrin can carry vesicles between the trans-Golgi network and
- lysosomes
- endosomes
- plasma mem.
vesicle fusion is mediated by
interactions between SNARE proteins on vesicles and target mem.
vesicle fusion is regulated by
Rab- GTP binding and hydrolysis
transport is inhibited by drugs that prevent
microtubule polymerization
vesicular transport is _______ dependent
calcium
fusion of what type of secretory vesicles with the plasma mem. is dependent on a specific extracellular signal?
regulated
the specific extracellular signal triggers the
release of calcium within the cell
calcium is AKA
the second messenger
what moves transport vesicles through the cell
motor proteins
what is considered the ‘highway’ in vesicular transport
the cytoskeleton
name the two motor proteins
- dynein
- kinesin
dynein is responsible for
retrograde transport
kinesin is responsible for
anterograde transport
what is the main function of lysosomes
to digest material taken in by endocytosis
lumenal lysosomal proteins are
digestive
membrane lysosomal proteins are
proton pump
mutations in the genes that encode for degradative enzymes found in lysosomes are responsible foe what 2 diseases
- Gaucher
2.Tay-sachs
the degradative enzymes of a lysosome are active at
acidic pH
how is the acidic pH maintained within a lysosome
by a proton pump
what are the degradative enzymes found in lysosomes called
acidic hydrolases
describe phagocytosis
phagosomes take up bacteria, old cells, and debris
phagosomes merge w/ __________________ for degradation
lysosomes
describe autophagy
cells’ own components are targeted for digestion
autophagy is important in certain stages of
embryonic development and programmed cell death
what types of cells would you expect to be engaged most heavily in endocytosis?
- macrophages
- NKC
- liver cells (glucose & cholesterol intake)
which types of cells would you expect to be engaged most heavily in exocytosis?
- neurons
- pancreatic cells
- liver cells
