Endoplasmic Reticulum resembles as Parking Garage

Endoplasmic Reticulum(ER) is one of the most important eukaryotic organelle known as the site of protein synthesis. This protein-making factory within the cells is studded with molecules (particularly known as ribosomes) that make proteins. A recent study was published in July 18th 2013 by Cell Press in the journal Cell, where researchers have redefined a new microscopic imaging method to understand how exactly ER sheets are stacked, revealing the three dimensional structure of the sheets like a helical parking garage.

Fig: original parking garage model of the 3D structure of ER sheets

Endoplasmic Reticulum is known to form stacked membrane sheets, an arrangement which is highly required to allow maximum membrane-bound polysomes for secretory protein synthesis.
 Earlier it was totally unknown until the recent picture reveals how these membranes are stacked. ER sheets are stacked one top of another in series with constant cytoplasmic and luminal spacing. It’s been 60years ER stacked sheets has been discovered, when Keith Porter who first used thin-sectioning electron microscopy to have clear view on tissues. Soon after that George Palade, Don Fawcett and others highlighted wonderful pictures of stacked rough ER which soon came to different cell biology textbooks. After these several years in 2013 Mark Terasaki of the University of Connecticut Health Center and his colleagues has come up with new 3D images which clearly depicts the new picture of the arrangement of the ER.


Fig: The Parking Garage model of stacked ER sheets where ribosomes are parked around the membrane. 

In this new experiment, Jeff Lichtman (principal researcher) and colleagues at Harvard used improved staining and automated ultrathin sectioning electron microscopic techniques to clearly analyze stacked ER sheets in neuronal cells and secretory salivary gland cells of mice. There is some point of reason behind the generation of stacked ER. ER has two lipid bilayers that help in maintaining narrow space over long distance. Although several experiments are performed, it remained unclear how ER sheets are stacked on top of another. Hence, a high resolution 3D actual picture was necessary to understand the mechanism of ER stacking.  This experiment gave a new turning point while using new scanning electron microscopic method where ER sheets are in continuous membrane system with stacked levels connected by a helicoids, which resembles a spiral staircase.

Conclusion:
In this theoretical model, named “parking garage” structure reflects the stacked ER sheets, helping in the increased number of ribosomes which perform protein synthesis. When the cell is in need of more protein, then it can reduce distances to pack more membrane in between. This model hence indicates the structure corresponds to a minimum of elastic energy of sheet edges and surfaces, allowing dense packing of ER sheets in the confined space of the cell. Since the helicoidal connections are observed in both neuronal cell bodies and in secretory cells of the salivary gland so it can be confirmed as a general feature of the cells. It is still now understood that this helicoidal connections might be present in cells where there is abundant stacked ER sheets like in secretory cells of pancreas or salivary gland. This may account for about 90% of secretory proteins among all synthesized proteins.

Source:
·         Mark Terasaki, Tom Shemesh, Narayanan Kasthuri, Robin W. Klemm, Richard Schalek, Kenneth J. Hayworth, Arthur R. Hand, Maya Yankova, Greg Huber, Jeff W. Lichtman, Tom A. Rapoport and Michael M. Kozlov. Stacked Endoplasmic Reticulum Sheets Are Connected by Helicoidal Membrane Motifs. Cell. 154: 285-296.

·  Another beautiful helix for biology, this time reminiscent of a parking garage. [http://www.eurekalert.org/pub_releases/2013-07/cp-abh_1071213.php]