The endoplasmic reticulum is an extensive membrane network of cisterna,
sac like structures held together by the cytoskeleton.
The phospholipid membrane encloses a space,
the luminal space from the cytosol.
The two varieties of ER are called rough endoplasmic reticulum,
and smooth endoplasmic reticulum.
The slide shows EM analysis of mouse
embryonic fibroblast and shows extended tubules of the rough ER.
The surface of the rough endoplasmic reticulum is studded with
protein manufacturing ribosomes giving it a rough appearance.
Ribosomes only bind to the ER once it
begins to synthesize a protein destined for sorting.
The membrane of the rough endoplasmic reticulum is
continuous with the outer layer of the nuclear envelope.
Although there is no continuous membrane between the rough ER and the Golgi apparatus.
Membrane bound vesicles are shuttle proteins between these two compartments.
The rough endoplasmic reticulum works in concert with the Golgi
complex to target new proteins to the appropriate destination.
Smooth endoplasmic reticulum functions in several metabolic processes,
including synthesis of lipids,
metabolism of carbohydrate, and calcium homeostasis.
It is connected to the nuclear envelope.
The smooth ER is best known for its storage of calcium ions in muscle cells.
The endoplasmic reticulum serves many general functions,
including the facilitation of protein folding and the transport of synthesized proteins.
Phospholipid synthesis, cholesterol and steroids synthesis,
detoxification reaction in control of many cellular signaling processes,
including calcium homeostasis, regulation of gene expression,
apoptosis, communication with mitochondria and the plasma membrane.
The ER membrane is one of
the major calcium storage organelles equipped with a number of
calcium transport proteins and
several calcium buffering proteins localized in the lumen.
ER luminal calcium concentration is relatively
high and could be as high as two to three millimolar,
where cytoplasmic calcium concentration resting
cells is approximately 50 to a 100 nanomole.
Calcium in the luminal of ER is bound to ER resident calcium buffering proteins.
Calcium is released from the ER by IP3R in response to hormonal stimulation of the cell.
Calcium released from the ER is sensed by STIM,
which transmits this information to plasma membrane calcium channels,
responsible for so-called store operated calcium channel.
Calcium is removed from the cytoplasm by SERCA endoplasmic sarcoplasmic calcium ATPase.
This talk will focus on two major functions of ER synthesis and folding of proteins,
and an ER role as a regulator of calcium homeostasis.