Tastefully Toasted is located at the unassuming corner of two wide, western streets in Boulder, Colorado. The product it caries may be full of taste, but is neither tasteful, nor toasted. This shop serves deep fried donuts, vulgarly smothered in a bevy of glazes and toppings; perhaps the most unique and amazing donuts I have ever tasted. Each donut is made to order, from batter to box right in front of your eyes. You can have a maple glazed graham cracker donut and an apricot glazed snickers donut and any other combination you can imagine. As I watched the creation of my donuts, I couldn't help but salivate at the scientific analogies, in this case, to the secretory pathway of a cell. The express purpose of the cellular secretory pathway is to synthesize and package proteins for release from a cell. Some familiar secretory proteins include hormones and neurotransmitters.

The Tastefully Toasted donut is born when batter is squeezed through a nozzle and drops into a vat of frying oil. In the secretory pathway, ribosomes translate an mRNA template into a chain of amino acids and push the growing protein through small pores into the Endoplasmic Reticulum (ER). Just as the oil cooks the batter into a crispy donut, the specialized pH environment and accompanying chaperones within the ER provide the perfect atmosphere for our young protein to fold, allowing it to establish the 3D structure which will define its function. An uncooked donut is just a blob of batter, and an unfolded protein is just a string of amino acids: characterless, bland, inedible.

After the donut reaches the perfect tinge of golden brown, it is transported slowly out of the oil on a conveyor belt, and the donuts make there way to the topping station. After a protein passes through the folds of the ER, it is packaged into small vesicle membranes and transported to the Golgi Apparatus, another membrane bound compartment and the location for most post-translational modifications -- in other words, the glazes and toppings. Here the donut is met by the donut chef, who individually dips each donut into its own glaze flavor and then coats the top half of the donut with the prescribed topping.

In the secretory pathway, special enzymes use specific hallmarks of the amino acid sequence to modify the protein. Even the seemingly small addition of a few Nitrogen atoms can be vital to establishing the flavor of a protein. In fact, sugars are added to both protein and donuts in their modifications. Scissor enzymes can also cleave the protein to remove pieces unnecessary to the final product or create two smaller proteins from an initially longer sequence. Although I didn't see it happen to my donuts, the donut chef would certainly have removed any burned sections, and probably cut my donut in half if I wanted him to.

After each donut is finished, it is placed in a special box, impervious to any and all sugary dripping, and sealed tight once all ordered donuts are present. After modification in the Golgi, the proteins are packaged into secretory granules, dense vesicles that travel to the edge of the cell membrane, where they wait for an external signal to trigger the release of their contents. My donuts sat in their box for some time before I ate them. It was morning when they were made, but they looked more like desert to me. When the urge called though, I popped open the box to find my array of detectible donuts. Even two days later, the last donut was still quite good, just as peptides in secretory granules can stay cued before their release is signaled. Retracing to the start of the process, each TT donut is made to order. In a similar way, although the release of secretory proteins is at the level of the secretory granules, up-regulation of DNA transcription into mRNA can call in an order for more of a specific protein.

To be fair, there are many differences between these two processes, one of the largest being that the secretory pathway is a factory sized operation as opposed to the individualized nature of the TT experience. Tastefully Toasted is also a truly one-of-a-kind shop, potentially the only place on earth that you can buy a hazelnut glazed granola donut. This stands in contrast to the millions of cells in each and every one of our bodies that enact the secretory pathway each day of our lives.

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