Download MBBS Trafficking of Protein III Lecture PPT

Download MBBS (Bachelor of Medicine and Bachelor of Surgery) Latest Trafficking of Protein III Lecture PPT



? A number of proteins



possess the amino acid sequence KDEL

(Lys-Asp-Glu-Leu) at their carboxyl terminal

? KDEL-containing proteins first travel to the GA in vesicles

coated with coat protein II (COPII)

? This process is known as anterograde vesicular transport.
? In the GA they interact with a specific KDEL receptor

protein, which retains them transiently.

? They then return to the ER in vesicles coated with COPI

(retrograde vesicular transport), where they dissociate from
the receptor, and are thus retrieved

? Certain other non-KDEL-containing proteins also pass to the

Golgi and then return, by retrograde vesicular transport, to the
ER to be inserted therein.
THE ER FUNCTIONS AS THE QUALITY

CONTROL COMPARTMENT OF THE CELL

? After entering the ER, newly synthesized proteins

attempt to fold with the assistance of chaperones and
folding enzymes.

? Some Chaperones and Enzymes Involved in Folding

That Are Located in the Rough Endoplasmic
Reticulum:

1. BiP (immunoglobulin heavy chain binding protein)
2. GRP94 (glucose-regulated protein)
3. Calnexin
4. Calreticulin
5. PDI (protein disulfide isomerase)
6. PPI (peptidyl prolyl cis-trans isomerase)
1. Calnexin

? The chaperone calnexin is a calcium-binding protein
? located in the ER membrane.
? This protein binds a wide variety of proteins,

including major histocompatibility complex (MHC)
antigens and a variety of plasma proteins.

? Calnexin binds the monoglucosylated species of

glycoproteins that occur during processing of
glycoproteins, retaining them in the ER until the
glycoprotein has folded properly


2. GRP94 (glucose-regulated protein)
? It is the most abundant protein in the ER lumen, and

is ubiquitously present in nucleated cells.

? GRP94 function as molecular chaperones and can

bind to malfolded proteins and unassembled
complexes.

? They are induced in response to stress, but once the

stress is removed the GRPs are posttranscriptionally
modified into biologically inactive forms.
3. Calreticulin:

? Calreticulin, which is also a calcium binding protein,

has properties similar to those of calnexin

? But it is not membrane-bound.
4. Protein disulfide isomerase (PDI):
? Protein disulfide isomerase (PDI) promotes rapid

formation and reshuffling of disulfide bonds until the

correct set is achieved

5. Peptidyl prolyl isomerase (PPI):
? It accelerates folding of proline-containing proteins

by catalyzing the cis?trans isomerization of X-Pro

bonds, where X is any amino acid residue.


? Misfolded or incompletely folded proteins interact

with chaperones, which retain them in the ER and

prevent them from being exported to their final

destinations.

? If such interactions continue for a prolonged period of

time, the misfolded proteins are usually disposed of

by endoplasmic reticulum associated degradation

(ERAD).

? This avoids a harmful build-up of misfolded proteins.
? In a number of genetic diseases, such as cystic

fibrosis, retention of misfolded proteins occurs in the

ER, and in some cases, the retained proteins still

exhibit some functional activity


Conformational Diseases That Are Caused by Abnormalities in



Intracellular Transport of Proteins and Enzymes due to Mutations
Endoplasmic Reticulum Stress

? Maintenance of homeostasis in the ER is important

for normal cell function.

? The unique environment within the lumen of the ER

is disturbed

o changes in ER Ca2+,
o alterations of redox status,
o exposure to various toxins or
o some viruses

can lead to reduced protein folding capacity and

the accumulation of misfolded proteins

? The accumulation of misfolded proteins in the ER is

referred to as ER stress.


? The cell responds to ES by unfolded protein response

to restore the ER homeostasis.

? The unfolded protein response is initiated by ER

stress sensors responds in many ways:

1. Transient inhibition of translation to decrease the

protein load entering the ER.

2. Increased expression of chaperons to enhance

protein folding.

3. Increased synthesis of protein required for

degradation of protein.

? If the ES persists, cell undergoes apoptosis.


? Degradation of misfolded proteins occur after these

proteins are transported back across the ER into
cytosol. (retrotranslocation or dislocation)

? Proteins are degraded in two ways -
A. By lysosomal proteases ? which do not require ATP
B. By Proteasome ? the proteasomal degradation

requires ubiquitin and ATP.

The protein to be degraded are marked by

attachment of ubiquitin .

It is major pathway of protein degradation.

This post was last modified on 30 November 2021