The Repressive ER

Often over-shadowed by other organelles such as the nucleus, chloroplast or vacuole, the endoplasmic reticulum (ER) – ‘an organelle of cells in eukaryotic organisms that forms an interconnected network of membrane vesicles’ – is slowly revealing its secrets (John Runions).

As a major component of the cell’s secretory pathway, the ER is intimately involved in protein synthesis via the ribosomes that are studded along its cytoplasmic surface (and which give rise to RER – rough endoplasmic reticulum). The process of protein synthesis is known as translation as it involves ‘translation’ of the message encoded in the m(essenger)RNA (which is itself made within the nucleus and carries the information for a particular protein originally contained within a gene in the cell’s DNA). However, once made, the ‘proteins’ – strictly speaking they are polypeptides: protein is a name that should be reserved for the fully functional, final product – are often altered to produce the protein, a process called post-translational modification.

While the details are beyond the scope of this item, the controls over gene expression – which include transcription, mRNA processing, and translation – are numerous. But one such system uses micro-ribonucleic acids (miRNAs), short-lengths of RNA that interact with mRNA thereby preventing its subsequent translation into protein. Now, here’s the take-home message: Shengben Li et al. have demonstrated that translation-inhibition activity by miRNAs occurs on the ER, and requires ALTERED MERISTEM PROGRAM1 (AMP1), which encodes an integral membrane protein associated with ER. But! Not only is this study important in identifying a previously unknown function of the ER, the work was performed in arabidopsis (i.e. a plant!), and, according to Xuemei Chen (lead researcher of the work), ‘as AMP1 has counterparts in animals, our findings in plants could have broader implications’. How refreshing to see plant work paving the way for animal/biomedical studies!

[For more on the world of small RNA, timely news that the Plant Cell’s Teaching Tools in Plant Biology on that topic has just been revised. This FREE resource – which includes a ready-made PowerPoint presentation, lecture notes, and teaching guide – can be accessed at http://www.plantcell.org/site/teachingtools/TTPB5.xhtml. – Ed.]