lecture series 6 part 2

7/24/2019 Lecture Series 6 Part 2

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Protein sorting and the Golgi

apparatus


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The Golgi Apparatus

Because of its large and regular structure, the Golgi apparatus

was one of the first organelles described by early light

microscopists.

It consists of a collection of flattened, membrane-enclosed

cisternae, somewhat resembling a stack of pancakes. Each of these

Golgi stacks usually consists of four to six cisternae

Each Golgi stack has two distinct faces a cisface !or entry face"

and a transface !or exit face". Both cisand transfaces are closely

associated with special compartments, each composed of a network

of interconnected tubular and cisternal structures.


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Transport of proteins from ER to

Golgi

Proteins destined for the Golgi, lysosome, PM, or extracellular fluid are packaged into vesicles at specialized

sites referred to as ER EXI !IE!"

ER exit sites are studded #ith receptors #hich $ind to proteins destined to leave the ER" Proteins leaving the

ER contain specific amino acid se%uences #hich are $ound $y these receptors"

&inding the receptor induces vesicle $udding and the transport of the vesicle to the cis'Golgi net#ork" It is

important to note that only properly folded proteins are transported"

(ollo#ing vesicle $udding, vesicles fuse to form a vesicular tu$ular cluster #hich is then transferred to the

Golgi"


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The ER retrieval pathway

)uring the vesicular transport of proteins from the ER to the Golgi, proteins from the ER can $e accidently

packaged #ithin the vesicles destined for the Golgi" Proteins resident to the ER are recovered $y the ER

RERIE*+ P+-.+/ 0RER1GR+)E R+2!P1R3" ER proteins are packaged in 41PI vesicles and

transferred $ack to the ER" Mem$rane proteins are easily packaged into the vesicle $y a 55XX se%uence"

!olu$le proteins, such as &ip, also contain retrieval signals ho#ever the mechanism is slightly different" hissignal consists of ys'+sp'Glu'eu 05)E se%uence3

!olu$le ER proteins #hich have escaped the lumen of the ER are retrieved $y 5)E receptors"

he affinity of 5)E receptors for 5)E se%uences is dependent on the p- of each organelle" .hile the

5)E receptor has a high affinity for the 5)E se%uence at the more acidic p- of the Golgi lumen, the

neutral p- of the ER lumen decreases the affinity of the receptor for the protein prompting its release"

hus the Retrieval Path#ay is p- dependent


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Golgi are involved in the sorting and post-translational

modification of proteins

)uring the passage of proteins through the Golgi compartments, various covalent

modifications take place in order to provide the specific structure and function to the

protein

Modification of existing glycosyl groups, O'glycosylation, sulfation 0addition of sulfates

to 1- on tyrosine3, and phosphorylation all take place #ithin the various Golgi

compartments"

(or simplicity, the primary focus of this lecture series #ill $e the modification of proteins

$y glycosylation

+s mentioned previously, the ERN'glycosylates various proteins #ith oligosaccharides

he Golgi then modifies these oligos providing either a 41MPEX

1IG1!+44-+RI)E and -IG- M+221!E 412+I2I2G 1IG1!+44-+RI)E" he high mannose oligo is produced $y removing glucose and 2'acetylglucosamine moeities #hile the

complex oligo is produced adding addition monosaccharides consecutively"

!ome proteins re%uire additional oligosaccharides to provide a specific function" he

Golgi also modifies proteins $y O'glycosylation" !erines are used for this type of post'

translational modification"


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Production of complex oligosaccharides

+s illustrated a$ove, theO-glycosylation andN'glycosylation in the Golgi is very complex involving

various types of mannosidases, glucosidases, glycosyl transferases, and monosaccharides"

Monosacchardes such as 2'acetylglucosamine, mannose, galactose, and 2'acetylneuraminic acid

02+2+3 are generally utilized for theN-glycosyl group modification" *arious nucleotides are

utilized to activate these sugars 06)P is the most dominant ho#ever 4)P can also participate3"

2umerous proteins depend on glycosylation for their uni%ue functions such as

PR1E1G/4+2!, heavily glycosylated proteins #hich are re%uired to form a fully functional

extracellular matrix"

1the proteins, such as acid hydrolases, rely on modifiedN'glycosyl groups for the targeting of the

protein to the lysosome"


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UDP-galactose transport

#$% #&%-galactose

#$% #&%-galactose

cytosol

Golgi lumen

protein-galactose

protein

galactosyltransferase

#&%

phosphatase

'()

%i

#&%-galactose#$% permease

Nand O'glycosylation are regulated $y nucleotide phosphatases located in the lumen of the Golgi"

Phosphatases are re%uired for the removal of the 7'phosphate from the nucleotide follo#ing glycosyl group

transfer"

he resulting nucleotide monophosphate is then utilized for the uptake of monosaccharides charged #ith

an 2)P"

(or instance 6)P'galactose uptake $y the Golgi is driven $y the export of 6MP" he charge difference on

the 6MP and 6)P drive the antiport process


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annose-!-phosphate targeting to

lysosome

+cid hydrolases targeted to the lysosome $y a mannose'8'phosphate

residue" his modification takes place in the cis'Golgi net#ork"

Prior to modification theN'glycosyl group is modified to produce a

high mannose'containing oligo" Phosphate addition to the 48 position

is catalyzed $y 2'acetylglucosamine phosphotransferase" his enzyme

$inds to and recognizes a signal patch on the folded hydrolase" he 2'

acetyl glucosamine is then removed to produce a mannose'8'phosphate


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Transport of !P-containing proteins to the lysosome and

"-#ell disease

I'cell disease is a pathology characterized $y a sharp decrease in acid hydrolases in lysosome" he

lack of hydrolases in these organelles is due to a mutation in the gene encoding the 2'acetyl

glucosamine phosphotransferase

!imilar to the Retrieval Path#ay in the ER, the $inding of M8P in the Trans'Golgi and release in thelumen of the lysosome is p- dependent"

In addition, M8PR are recovered $y a Retrieval Path#ay in a similar manner as 5)E receptors"

lecture series 6 part 2

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