How does a protein get to the

correct cellular location? Membrane and organelle proteins

contain targeting (sorting) signals in their

amino acid sequence.

Targeting signals are recognized during or after the protein is translated - special

machinery recognizes the signal and

translocates the protein to its correct

location

Target Usual Signal Location

Signal

Removed? SIGNAL

ER N-terminal

Or internal

(+/-) 6-12 hydrophobic aa

often preceded by 1

or more (+) aas

Mitochondrial

matrix

N-terminal

(multiple)

(+) Amphipathic helix

20-50 residues with

R/K and

hydrophobic sides

Peroxisome

C-terminal (-) Usually S-K-L at C-

terminus

Nucleus Internal (-) 1 cluster 5 basic

aas or 2 smaller basic clusters

separated by 10 aa

Examples of protein targeting

signals

Proteins are targeted to different

compartments in different ways

Proteins that are

targeted to the

nucleus,

mitochondria,

chloroplasts and

peroxisomes are

synthesized on free

ribosomes as soluble

polypeptides

Proteins that are

targeted to the cell

surface, Golgi and

Lysosomes are

synthesized on

ER membrane

bound ribosomes

and move through

the secretory

pathway

The ER targeting mechanism

requires two special receptor

proteins:

What gets the ribosomes with

secretory protein mRNA's to bind

to the ER membranes?

1. Signal recognition particle (SRP)

2. SRP receptor

Translation of secretory mRNA

begins on free ribosomes

N-terminal signal sequence emerges

from ribosome tunnel

Signal recognition particle (SRP) binds

to the emerging signal

sequence from the

ribosome

SRP receptor initiates the interaction of

signal sequences with the ER membrane

Receptor is an a,b dimer b subunit is an intrinsic

membrane protein

a-subunit initiates binding of ribosome SRP to ER

membrane

How do intrinsic membrane

proteins get inserted into the

ER membrane?

Topologies of some integral membrane

proteins synthesized on the rough ER

Most cytosolic transmembrane proteins have

an N-terminal signal sequence and an

internal topogenic sequence

Type I protein

A single internal signal-anchor sequence directs

insertion of single-pass Type II transmembrane

proteins

Type II

protein, no

N-terminal

signal

sequence

Multipass transmembrane proteins have

multiple topogenic sequences

Most proteins synthesized in the

Rough ER are glycosylated by a

core oligosaccharide that is linked

to asparagine residues

(N-linked glycosylation)

The

glycosylation

signal is Asn-

X- (Ser/Thr)

N-linked glycosylation occurs

during protein translocation via

the membrane bound protein

oligosaccharide transferase

Core Glycosylation and Trimming

in the ER lumen

Overview

of secretory

pathway

ENDOPLASMIC RETICULUM

GOLGI

Transport

1.In

2.out

In:

-Early endosome

-Late endosome

-Golgi

-ER

Molecular mechanism of membrane transport

In and out transport

Protein pengikat/coated vesicles

-Clathrin (from golgi and from membrane)

-Copi (from golgi to ER)

-Copii (from ER to golgi)

Sebenarnya protein tersebut sangat beragam keberadaannya

Pembentukan clathrin vesicle

Tersusun atas triskelion protein clathrin

Multi subunit protein adaptin. Adaptin akan berikatan pada clathrin dan transmembran protein, transmembran reseptor, yang akan menagkap

molekul cargo sehingga disebut Reseptor Cargo. Sedikitnya ada 4 jenis

adaptin dan bersifat spesifik terhadap

Mekanisme terjadinya Budding, dibantu oleh:

1.Reseptor

2.Adaptin

3.Clatrin

4.Dynamin

5.Dynamin complex protein (merusak membran dan atau merubah lipid

membran

Clathrin coat akan dilepas dari budding, auxillin yang merupakan bagian dari vesicle akan mengaktifasi ATPase, HSP70 yang akan menghidrolisis

clathrin.

Mutasi dynamin tidak mampu membuat budding

Pengontrolan transport membran terjadi dengan teratur karena adanya spesifitias

Protein SNARE dan target GTPases (Rabs).

Ikatan SNAREs bersifat

Helix dan disosiasinya

dibantu oleh NSF (ATPase)

dan ATP

HIV menggunakan cara yang mirip dengan mekanisme docking dan integrasi

membran plasma

Pertanyaan:

Cara dan protein yang berperan dalam Pembentukan vesicle?

Mekanisme dan protein yang terlibat dalam budding?

Protein Pengawalan transport membran?

Fungsi ATP dan GTP dalam transport membran?

Kenapa transport membran bersifat spesifik dan teratur?

sebutkan letak dan fungsi protein SNARE, Sar1 dan Rab?

ER: proses maturasi protein

setelah disinteris oleh ribosom,

penambahan gula pada protein

Golgi:

Penambahan dan

modifikasi oligosakarida

kepada protein

Memproduksi karbohidrat

dan lemak

Mekanisme pembetukan vesikel di ER

lysosom:

- Acid hydrolase

- Memerlukan ATP

-