How many times have you dismissed a UFO (unidentified fluor-/chemiluminescent object) on your western blot as an uninteresting, accidental degradation product – focusing only on the band running at the predicted size for your protein of interest?
A research article published in Cell Communication & Signaling highlights the functional value of just such a UFO in the beating heart, detailing the mechanisms of its very deliberate production.
Researchers studying the major gap junction protein connexin-43 have for many years noticed a 20 kDa band (now known as GJA1-20k) on their blots. But what might have been written off as a UFO was recently shown by Smyth and Shaw to be a separate protein of independent function, the product of internal translation acting on connexin-43’s mRNA transcript (Cell Rep, 2013, Nov 14, 5(3): 611–618).
Internal translation of mRNA transcripts is a rare but long-studied phenomenon, with more than 100 known examples. Smyth and Shaw’s elegant study described how cells use internal translation to generate a protein equivalent to a C-terminal fragment of connexin-43, and that this truncated protein has an important function as a chaperone for full-length connexin-43 as it traffics to gap junctions. It is so important, in fact, that loss of connexin-43 internal translation results in the full-length protein becoming stuck in the endoplasmic reticulum.
The Smyth and Shaw study makes the assumption that GJA1-20k is produced by a cap-independent translation mechanism, as the literature often describes internal translation as an exclusively cap-independent process. However, Trond Aasen and colleagues from the Autonomous University of Barcelona, Spain, noticed that dissenting voices were emerging who argued against the ‘cap-independent only’ paradigm for internal translation (for example, in Gene, 2012, Jul 12, 502(2): 75–86).
In a Short Report published in Cell Communication & Signaling, Aasen and colleagues now convincingly demonstrate that GJA1-20k undergoes cap-dependent, and not cap-independent, translation. They further find that GJA1-20k translation is additionally dependent on upstream translation – reminiscent of a previously described bait-and-trap system for ribosomes – and that its abundance is negatively regulated by Mnk1/2 kinase signaling.
As connexin-43 and its function at gap junctions is associated with a number of pathological conditions (especially cardiac arrhythmias, but also skin, neurological and developmental disorders, as well as cancer), these new insights into the regulation of its trafficking chaperone are an important contribution to the study of a number of different diseases.
Written by Naomi Attar (@naomiattar), Associate Editor for BMC Biology.