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ENDOPLASMIC RETICULUM
INTRODUCTION
• ER is found in Eukaryotes.
• In plant cells, ER occurs as - system of membrane tubules and sheets
(cisternae) -distributed throughout the cytoplasm
• Two different types,
ER
Smooth ER
Rough ER
Mostly tubules
Cisternae, sheets
• Seen as a continuous network with nuclear envelope.
• Connected with the Golgi bodies.
ROLES
SER
RER
•
• Protein synthesis
• Post transitional modification of
proteins
PROTEINS
•
•
Metabolic processes
. Lipids
- Steroids
- Phospholipids
Metabolizes the carbohydrate
Detoxifying the drugs
cytoplasm
RER
both has ribosomes
end up in nucleus secreted into CM,
mitochondria,
integrated in
peroxisomes,
cytoplasm, ER,
Stay back.
golgi bodies,
lysosomes.
ER Dynamics and Mobility
ER tubular extension
1) Sliding dynamics(SD)
2) Tip Attachment Complex(TAC)
SD : By sliding of ER tubules along the microtubule (MT) cytoskeleton
More predominant and faster.
TAC : Physical interaction b/w ER Resident protein (STI M1)
And
MT plus end binding protein (EB1)
STI M1 has the MT binding domain ( STI M1 --- ER tubule movement)
ER-MT connections for ER remodeling.
Rab 10- ER specific Rab GTPase – ER tubule extension along MT.
used as a marker to find the new ER tubule growth.
Shaping ER tubules
• Reticulon (RTN) and DP1/Yop1p family members
(RHD3)
closely spaced TMD’s (Trans-membrane Domains)
RTN and DP1/Yop1p are required for
1) Shaping of tubules
(network formation)
2) Tubule formation in cos-7 cells
3) Responsible for curved edges of ER sheets and nuclear pores
Overexpressed
Induces constrictions
results
so from cisternae to tubules
highly curvature membrane
Three way junctions
The ER network of COS-7 cells is labeled with mCherry-KDEL (red) and the junctions
are labeled with Lnp1-GFP (green).
Three way junction:
• GTPase is the mediator for the formation of the network junctions.
Sey 1p (yeast) and RHD 3 (Arabidopsis)
plant homologue of atlastin
Lnp1p (protein of lunapark family) – localises the ER network in yeast and
mammalian cells.
binds to
Reticulon / Yop1p
resides @ ER tubule junctions (stabilizes)
ER movement in plants  Actin – Myosin dependent
XI-K
XI-1, XI-2
a)
b)
ER
PM
APC
actin
ER
myosin
Tubule
c)
Fusion
complex
Cisternae
a) Tubule growth or retraction
b) Anchor points
c) Tubule fusion and three way junction formation
d) Tabulation versus cisternalisation (due to displacement of RTN’s)
APC- Anchor Point Complex
Fusion/tethering complex – RHD3
d)
Vesicle formation
• COP I and COP II – Vesicle coat protein occurs @ TER
ER
proteins –(COP II)
Golgi Apparatus
*TER– Transitional ER
ERGIC- ER Golgi Intermediate complex
TGN- Trans Golgi Network
Cortical MT associated ER sites
• Cortical ER network and its nearby MT’s intersect with each other.
signal transduction , movement of molecules
* ER membranes -- supported by Actin network – Myosin motors
• This interaction is CMER’S (anchoring points)
pausing them
organelles meet, exchange of components
MT
MT
CM
PM
ER
a)
b)
Myosin
Actin
Golgi
MT
C
ER
Kinesin
The ER membranes are associated with actin filaments that drive organelle
movements along the membrane (e.g. Golgi; or other compartment carrying
cargo (C)) with support of myosin motor proteins. Organelles may carry MTassociated proteins (e.g. kinesin) that cause pausing at C-MERs through
binding to the MTs.
Many plant viruses exploit C-MER’S for replication and PD
targeting
Summary
•
•
•
•
•
•
•
•
Network of membranes
Smooth and Rough ER
Roles – metabolizes , protein synthesis, post transitional modification
Tubule extension by sliding dynamics or TAC
Reticulons and Dp1/YOP 1p involved for shaping of ER
Three was junctions or network formations
COP II protein involved for vesicle formation
Cortical MT associated ER sites
References
• Pen E J , Heinlein M: Cortical microtubule-associated ER sites:
organization centers of cell polarity and communication. Curr Opin
Cell Biol 2013, 16:1–10.
• Sparkes I, Hawes C, Frigerio L: FrontiERs: Movers and shapers of
the higher plant cortical endoplasmic reticulum. Curr Opin Cell Biol
2011, 14:658–665.
• Chen S, Novick P, Novick S F: ER structure and function. Curr Opin
Cell Biol 2013, 25:1–6.
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