BCH3021 Lecture Notes - Lecture 17: Signal Recognition Particle, Endoplasmic Reticulum, Signal Peptidase

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25 May 2018

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Lecture 17 – Modification and Maturation of Secretory Proteins

Concepts

• Most polypeptide synthesized in ER become glycoproteins through addition of

branched carbohydrate chains to specific Asn residues

• Further modifications of these carbohydrate chains – addition of new chains to

specific Ser or Thr residues – occurs in Golgi

• Important elements of extracellular matrix – glycosaminoglycans and

proteoglycans

o Synthesised in Golgi

• To protect cells, some proteins do not acquire biological activity until late in

secretory pathway or beyond

o Activation of such proteins usually involved proteolysis

• Free ribosomes are unattached to any membrane

• Membrane bound ribosomes coat surface of ER → rough endoplasmic

reticulum

• All proteins are directed to ER by signal sequence of small hydrophobic

amino acids → binds to signal recognition particle → protein synthesis slows

down → SRP complex binds to SRP receptor → SRP is released and protein is

passed to protein translocation in membrane

• Signal sequence also opens translocation channel → signal peptide remains

bound to channel → protein passed through into lumen → signal sequence is

cleaved off by signal peptidase located on luminal side of membrane →

protein released id then degraded

o N-terminal signal sequence initiates translocation

• Some proteins remain embedded in ER as Transmembrane proteins

• Stop transfer sequence is hydrophobic

o Released laterally from translocation channel

o Internal signal sequence is used to start the protein transfer until a stop

sequence is reached

Overview of Secretory Pathway

Most Secreted Proteins are Glycosylated

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• Glycosylation is attachment of linear or branched carbohydrate chains to

proteins synthesized in secretory pathway

• N – linked glycosylation starts in ER with further modification in Golgi

• O – linked glycosylation occurs in Golgi

• Glycosylation

o Promote folding or prevent aggregating

o Confer resistance to degradation

o Promote binding to other proteins

N-linked and O-linked Oligosacchardies have Distinct Structures

• N-linked oligosaccharides

o Added in ER and further modified in Golgi

o Coupled to protein via Asn that is part of motific Asn-Xxx-Ser/Thr

o Are complex – up to 16 residues

o Initially added as a pre-synthesized block of residues

o Present on glycoprotein

• O-linked oliogsaccharides

o Added in Golgi

o Coupled in protein via hydroxyl group of Ser or Thr (simpler)

o Are short – one to four residues and are added one at a time

N-linked Sugars are Made in the ER

• On cytoplasmic face of ER membrane oligosaccharides are added unit by unit

onto carrier lipid, dolichol

• First unit is attached to dolichol by phosophate bridge

• Branched Man5GlcNAc2 lipid intermediate is flipped across membrane by a

transporter protein

• 4 more mannoose and 4 glucose residues are added in ER to form

Glc3Man9GlcNAc2-dolichol precursor

N linked Oligosaccharides are transferred to Unfolded Nascent Protein in ER

• GlcMan9GlcNAc2 is transferred from dolichol precursor to unfolded nascent

protein

• Enzyme moves oligosaccahrdies and attaches it to Asparagine (Asn)

o Occurs as protein is being synthesized

Further processing of N-linked Oligosaccharides in

ER is integrated with Protein Folding machinery

• Glc3Man9GlcNAc2 – transferred from

precursor to unfolded nascent protein

• 2 of 3 Glc residues are sequentially removed

to leave 1 remaining

• If glycoprotein is folded correctly, final Glc

is removed and 1 man is removed → protein

moves to Golgi

• If protein is not folded correctly, 1 Glc is

added back to enhance binding to folding

chaperones calrecticulin or calnexin

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Document Summary

Lecture 17 modification and maturation of secretory proteins. N linked oligosaccharides are transferred to unfolded nascent protein in er: glcman9glcnac2 is transferred from dolichol precursor to unfolded nascent protein, enzyme moves oligosaccahrdies and attaches it to asparagine (asn, occurs as protein is being synthesized. Er is integrated with protein folding machinery: glc3man9glcnac2 transferred from precursor to unfolded nascent protein, 2 of 3 glc residues are sequentially removed to leave 1 remaining. If glycoprotein is folded correctly, final glc is removed and 1 man is removed protein moves to golgi. If protein is not folded correctly, 1 glc is added back to enhance binding to folding chaperones calrecticulin or calnexin: example of carbohydrate which plays a role in proteins transport. Ecm: proteoglycans are built in er and golgi, core polypeptide is synthesised on rough er and translocated into lumen, specialized polysaccharide chains are built on core polypeptide in.

Related Questions

QUESTION 30

The function of centrosome is:

	1.	Microtubule initiation and organization in animal cells
	2.	Organization of thin filaments
	3.	Depolymerization of microtubules
	4.	Organization of intermediate filaments

2.5 points

QUESTION 31

The destinations of proteins synthesized on "free" ribomsomes include:

	1.	Endoplasmic reticulum
	2.	Nucleus
	3.	Mitochondria
	4.	Lysosomes

2.5 points

QUESTION 32

Microtubules can be assembled and disassembled quickly in response to the cellular changes is because:

	1.	Cells synthesize tubulin proteins very fast.
	2.	Cells can turn other cytoskeletal filaments into microtubules
	3.	Microtubules have an instable plus end and microtubules are probably organized by weak non-covalent interactions
	4.	The motor proteins of microtubules can organize the microtubules in response to the cellular changes

2.5 points

QUESTION 33

Both signal recognition particle and a translocon in rough endoplasmic reticulum membrane can recognize and bind to the signal sequence of a secretory protein, so they play the exact same role during protein co-translational translocation.

True

False

2.5 points

QUESTION 34

Rough endoplasmic reticulum is so named is because:

	1.	It has many proteins associated with its surface.
	2.	It has ribosomes associated with its cytosolic surface.
	3.	It is associated with nuclear membrane.
	4.	It has vesicles budding off its surface.

2.5 points

QUESTION 35

The membrane vesicles that transport materials between membrane compartments have a protein coat. The protein coat has similar functions in different vesicles. These functions include:

	1.	Help the enclosed material fold correctly.
	2.	Help degrade the protein in the vesicles
	3.	Maintain the internal pH of the vesicles
	4.	Function as mechanical device to form a vesicle and select the components to be carried by a vesicle.

2.5 points

QUESTION 36

Lysosomes are important for:

	1.	Receptor-mediated endocytosis.
	2.	Breaking down foreign particles brought into the cells by endosomes
	3.	Regulating the process of organelle turnover.
	4.	Protein trafficking between membrane compartments

2.5 points

QUESTION 37

Which of the following is correct about membrane biosynthesis?

	1.	Membranes are synthesized in Golgi Complex.
	2.	Membranes arise from preexisting endoplasmic reticulum membranes.
	3.	Membranes are synthesized on ribosomes.
	4.	Membranes of all organelles have the same composition of proteins and lipids, since they all originate from endoplasmic reticulum.

2.5 points

QUESTION 38

During skeletal muscle contractions, the shortening of the muscle fibers is because of:

		The shortening of the thin and thick myofilaments.
		The shortening of the cytoskeletal filaments.
		The kinesin proteins moving over microtubules.
		The actin and myosin filaments sliding over each other.

2.5 points

QUESTION 39

The assembly and disassembly of microtubules is regulated by rate of growth and shrinkage of the plus end.

True

False

2.5 points

QUESTION 40

The functions of Golgi complex include:

	1.	O-linked oligosaccharides and most of a cellâs complex polysaccharides are assembled in Golgi complex
	2.	Cis Golgi complex sorts proteins
	3.	Trans Golgi complex segregates proteins in different types of vesicles for dilevery.
	4.	N-linked oligosaccharides are further processed in Golgi complex

ochrecoyote721

The vesicles that transport materials from donor membranes to acceptor membranes include:

	1.	COP-II coated vesicles
	2.	COP-III coated vesicles
	3.	COP-I coated vesicles
	4.	Clathrin coated vesicles

2.5 points

QUESTION 11

During skeletal muscle contraction, the roles of ATP include:

	1.	ATP is not required for forming the crossbridges between myosin and actin filaments.
	2.	ATP is utilized as a structural linker to mediate the interaction between myosin and actin filaments
	3.	Hydrolysis of ATP provides energy for myosin head to move along actin filaments and the binding of ATP to myosin head allows the detachment of myosin fromactin filaments.
	4.	ATP binds to actin filaments to drive the power stroke of myosin head.

2.5 points

QUESTION 12

One of the functions of smooth endoplasmic reticulum is

	1.	Synthesizing steroid hormone in the endocrine cells of the gonad and adrenal cortex
	2.	Serves as the starting point of the biosynthetic pathway
	3.	Synthesizing secretory proteins
	4.	Degrading cell debris

2.5 points

QUESTION 13

The function of COP-II coated vesicles is mainly:

	1.	Transporting materials from endosomes to Golgi complex
	2.	Moving materials from the ER to the ERGIC and Golgi complex.
	3.	Transporting materials from Golgi complex back to endoplasmic reticulum.
	4.	Involved in receptor-mediated endocytosis.

2.5 points

QUESTION 14

Which of the following processes is completed in rough endoplasmic reticulum?

	1.	Chaperones help misfolded proteins fold correctly.
	2.	Proteins that target to nucleus are synthesized in rough endoplasmic reticulum.
	3.	O-linked glycosylation occurs in rough endoplasmic reticulum.
	4.	Misfolded proteins are destructed in rough endoplamic reticulum.

2.5 points

QUESTION 15

Kinesin is the motor protein that coorporates with microtubules. It has the following functions:

	1.	It uses GTP as energy source.
	2.	It can hydrolyze ATP and use the energy to drive cargo along microtubules from minus end to plus end.
	3.	It has a globular head that can act as GTPase
	4.	The tail of kinesin is more diverse and binds to cargo

2.5 points

QUESTION 16

For correct segregation and trafficking, lysosomal proteins are tagged in the cis-Golgi with phosphorylated mannose residues, and then the tagged lysosomal enzymes are recruited to clathrin coated vesicles by mannose 6-phosphate receptors.

True

False

2.5 points

QUESTION 17

The process that specialized cells, such macrophages, engulf relatively large particles is:

	1.	Called pinocytosis
	2.	Called phagocytosis
	3.	Mediated by COP-II coated vesicles
	4.	Called autophagy

2.5 points

QUESTION 18

Proteins synthesized on rough endoplasmic reticulum and smooth endoplasmic reticulum are imported co-translationally to their destinations

True

False

2.5 points

QUESTION 19

Treatment of cells with a drug that promotes microtubule disassembly disperses Golgi complex into separate stacks of membrane compartments, suggesting that:

	1.	Golgi complex is made of microtubules.
	2.	Microtubules are important for the organization of Golgi complex.
	3.	Microtubules are transported into Golgi complex
	4.	Golgi complex is composed of tubulin proteins

2.5 points

QUESTION 20

Which of the following is correct about signal sequence of a secretory protein?

	1.	It is an âaddress codesâ for protein trafficking pathways
	2.	It is usually located at the C-terminus.
	3.	It targets the ribosome and the protein that is being synthesized to endoplasmic reticulum.
	4.	It is removed in Golgi complex.

apricotgrasshopper117

Proteins that end up on the outside of the cell's plasma membrane (outer membrane) are produced by ribosomes attached to the endoplasmic reticulum (ER), and then the flow thorough the channels of the ER, where sugars get attached to their surfaces. Then they flow in the Golgi apparatus, where they are formed into vesices that travel to the cell membrane and fuse with it, becoming part of the outer membrane.

Protein X is a protein that gets to be produced y this pathway, and ends up on the outer surface of liver cells. Below is the structure of an exotic sugar that is found to be bound to tyrosine residues in thwo places on protein X. Using radioactively labeled carbons, it is detemined that all the carbons in this exotic sugar come from glucose, but the pathway is unknown.

a) Propose and write out a series of reactions (a pathway) by which this sugar could be formed, using chemical structures, and incluing all relevant substrates and co-subtrates as well as cofatos that you think are likely to beinvolved. (Try to use as few steps as possible, but hopefully no more than ten)

b) In addition, write out a reaction by which you think this exotic sugar would attach to the tyrosine residues on protein X. Show steps and structures invovled.

BCH3021 Lecture Notes - Lecture 17: Signal Recognition Particle, Endoplasmic Reticulum, Signal Peptidase

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