Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic functions in the cell

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SISTEM

ENDOMEMBRAN

KOMPARTEMEN INTRASELULER

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Concept 6.4: The endomembrane system regulates protein traffic and performs metabolic functions in the cell

• _{Components of the}_{endomembrane system}_:

– _{Nuclear envelope}

– _{Endoplasmic reticulum} – _{Golgi apparatus}

– _Lysosomes – _Vacuoles

– _{Plasma membrane}

• These components are either continuous or connected via transfer by vesicles

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The Endoplasmic Reticulum: Biosynthetic Factory

• _The_{endoplasmic reticulum (ER)}_{accounts for}

more than half of the total membrane in many eukaryotic cells

• _{The ER membrane is continuous with the}

nuclear envelope

• _{There are two distinct regions of ER:}

– _{Smooth ER}_{, which lacks ribosomes}

– _{Rough ER}_{, with ribosomes studding its}

surface

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Fig. 6-12

Smooth ER

Rough ER Nuclear

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Functions of Smooth ER

• _{The smooth ER}

– _{Synthesizes lipids}

– _{Metabolizes carbohydrates}

– _{Detoxifies poison}

– _{Stores calcium}

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Functions of Rough ER

• _{The rough ER}

– _{Has bound ribosomes, which secrete}

glycoproteins (proteins covalently bonded to carbohydrates)

– _Distributes_{transport vesicles,}_proteins

surrounded by membranes

– _{Is a membrane factory for the cell}

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• _The_{Golgi apparatus}_{consists of flattened}

membranous sacs called cisternae

• Functions of the Golgi apparatus:

– _{Modifies products of the ER}

– _{Manufactures certain macromolecules}

– _{Sorts and packages materials into transport}

vesicles

The Golgi Apparatus: Shipping and Receiving Center

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Fig. 6-13

cis face

(“receiving” side of

Golgi apparatus) _Cisternae

trans face

(“shipping” side of

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Lysosomes: Digestive Compartments

• _A_lysosome_{is a membranous sac of hydrolytic}

enzymes that can digest macromolecules

• Lysosomal enzymes can hydrolyze proteins,

fats, polysaccharides, and nucleic acids

Animation: Lysosome Formation

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• Some types of cell can engulf another cell by

phagocytosis; this forms a food vacuole

• A lysosome fuses with the food vacuole and

digests the molecules

• Lysosomes also use enzymes to recycle the

cell’s own organelles and macromolecules, a process called autophagy

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Fig. 6-14

Nucleus 1 µm

Lysosome

two damaged organelles 1 µm

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Vacuoles: Diverse Maintenance Compartments

• A plant cell or fungal cell may have one or

several vacuoles

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• _{Food vacuoles}_{are formed by phagocytosis}

• _{Contractile vacuoles}_{, found in many}

freshwater protists, pump excess water out of cells

• _{Central vacuoles}_{, found in many mature plant}

cells, hold organic compounds and water

Video: Paramecium Vacuole

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Fig. 6-15

Central vacuole

Cytosol

Central vacuole Nucleus

Cell wall

Chloroplast

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SISTEM

ENDOMEMBRAN

KOMPARTEMEN INTRASELULER

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Polyribosomes

• A number of ribosomes can translate a single

mRNA simultaneously, forming a

polyribosome (or polysome)

• _{Polyribosomes enable a cell to make many}

copies of a polypeptide very quickly

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Completing and Targeting the Functional Protein

• _{Often translation is not sufficient to make a}

functional protein

• _{Polypeptide chains are modified}_after

translation

• _{Completed proteins are targeted to specific}

sites in the cell

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Protein Folding and Post-Translational Modifications

• During and after synthesis, a polypeptide chain

spontaneously coils and folds into its three-dimensional (3D) shape

• _{Proteins may also require}_{post-translational}

modifications before doing their job

• _{Some polypeptides are}_activated_by_enzymes

that cleave them

• _{Other polypeptides come}_together_{to form the}

subunits of a protein

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Protein Folding

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Protein Folding

Figure 6–86 The Hsp70 family of molecular chaperones.

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Targeting Polypeptides to Specific Locations

• Two populations of ribosomes are evident in

cells: free ribsomes (in the cytosol) and bound ribosomes (attached to the ER)

• Free ribosomes mostly synthesize proteins that

function in the cytosol

• Bound ribosomes make proteins of the

endomembrane system and proteins that are

secreted from the cell

• Ribosomes are identical and can switch from

free to bound

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• Polypeptide synthesis always begins in the cytosol

• Synthesis finishes in the cytosol unless the polypeptide signals the ribosome to attach to the ER

• Polypeptides destined for the ER or for secretion are marked by a signal peptide

• A signal-recognition particle (SRP) binds to the signal peptide

• The SRP brings the signal peptide and its ribosome to the ER

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Ribosome

Sintesis protein pada REK:

• Translasi protein berawal di sitosol.

• Peptida sinyal (20-25 residu AA hidrofobik pada N-terminal) diikat oleh SRP.

• SRP berikatan dengan reseptor RSP (protein penambat/docking protein) yang ada di membran REK, menambatkan ribosom ke REK.

• SRP dilepaskan dan sintesis protein dilanjutkan.

• Polipeptida mengalami elongasi (nascent polypeptide) masuk ke dalam lumen REK dan peptida sinyal dilepaskan.

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Smooth ER

Nucleus

Rough ER

Plasma membrane cis Golgi

trans Golgi

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Modifikasi protein pada Retikulum Endoplasma

• _{Penambahan gugus}

fungsional tertentu pada protein.

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To be continue… Take home activity

1. Modifikasi protein pada RE

2. Sortasi protein sekresi pada apparatus Golgi