Just because a polypeptide has been translated from an mRNA sequence, doesn’t mean the protein is complete. In this lesson, you’ll learn about possible post-translational modifications, including phosphorylation, ubiquitylation, glycosylation, and cleavage.
Ok, so you took a shower and got dressed today in a simple pair of pants and a t-shirt.
That’s a good start. In some cases, that’s plenty. That’s good enough. But sometimes you’re going to need that little something extra. If you’re playing soccer today, for example, you’re going to need that pair of cleats and a cool headband. If you’re playing in the snow, a scarf, gloves, and fleece hat are kind of essential. If you were to, let’s say, do something completely extraordinary and go scuba diving today, well, you’re going to need a wetsuit.
And an oxygen tank.Let’s call these ‘extras’ post-dressing modifications. They are additional items you’d need to do something specific. Now let’s liken these to post-translational modifications.
I assure you, they are just as interesting as the something extra that you’d consider putting on to have fun today. They too specify certain activities or roles, only for proteins. Post-translational modifications are changes made to a polypeptide or protein any time after translation. Now, ‘Whoa, wait a minute!’ you say – after a polypeptide is made from RNA during translation, aka protein synthesis, it’s not complete? It needs something extra? Well, sometimes you’re fine in a t-shirt and pants. Sometimes, it’s not quite enough.
Post-translational modifications can include cleaving a polypeptide or adding something to it. In this lesson, we’ll talk about some of these modifications and why they help specify or change a protein’s function, much like how the finishing touches on your outfit might have a lot to do with what you’re going to do today. Post-translational modifications occur on both eukaryotic and prokaryotic proteins. However, not all the same ones occur in both cells. In this lesson, we’ll focus on the post-translational modifications that can happen to proteins specifically in eukaryotic cells.
Phosphorylation is a type of post-translational modification that adds a phosphate group to a protein. This is often added to a serine, tyrosine, or threonine amino acid residue. A kinase is the protein enzyme that adds this phosphate group. Not all proteins are phosphorylated. However, for some proteins, phosphorylation can activate or even sometimes inactivate a protein.
You heard that right – when a kinase phosphorylates a target protein, it can turn the protein ‘on’. However, there are some proteins that are actually turned ‘off’ by phosphorylation by a kinase. It depends on the protein that is being phosphorylated.Now, just as you might put on a pair of cleats to play soccer, you should definitely take them off when you’re off the field. You wouldn’t want to walk through the house in those spikes.
Similarly, phosphorylation can be reversed. Dephosphorylation is the removal of a phosphate group. A phosphatase is a protein enzyme that removes a phosphate group. Dephosphorylation would do the opposite of whatever phosphorylation did. So, if adding a phosphate activated a protein so that it could do a specific job, then dephosphorylation would deactivate a protein so that it would stop doing that job, and vice versa.
Ubiquitylation, also called ubiquitination, is a type of post-translational control that adds ubiquitin to a protein. A ubiquitin is a small regulatory protein that can have different functions. Ubiquitins are added to proteins by a specialized cascade of enzymes.Often, when a protein is modified with a chain of ubiquitin proteins, that ubiquitylated protein is targeted for degradation or destruction. It’s like putting a tag on a protein to signal that it’s time to take this protein out to the trash.
This would happen when a protein is no longer needed.Sometimes, a single ubiquitin is added to a protein as a signal. One example of this in the cell is the single ubiquitin mark added to some histones during chromatin modification. Remember that DNA is wrapped around these histones as a part of normal chromatin structure and that chromatin structure is modified to regulate levels of transcription – turning transcription on, off, up or down . Therefore, a single ubiquitin on some histones might serve as a signal to recruit specific transcription factors or proteins to a gene or to just regulate transcription.
In addition to transcription, a single ubiquitin protein can signal other processes as well.
In addition to phosphorylation and ubiquitylation, there are several other types of post-translational modifications – more than we can name here.For example, in addition to a single ubiquitin, histones and other proteins can be modified by the addition of methyl or acetyl groups.
Like a ubiquitin signal, this could help regulate the levels of transcription at a gene.Glycosylation is another type of post-translational modification that adds carbohydrates to a protein. This creates what we know as glycoproteins. These glycoproteins, usually modified with a chain of carbohydrate sugars, are often found in the cell membrane or secreted out of the cell.
Glycosylation occurs within the endoplasmic reticulum and the Golgi Complex.Lastly, there’s another type of modification that isn’t exactly adding anything at all, but rather, taking something off. Cleavage is a type of post-translational modification that involves cutting a polypeptide bond of a protein.
For example, you’ve probably heard of the protein collagen before. While some choose to inject collagen as a beauty supplement, collagen is naturally synthesized by your body as part of connective tissue. Collagen actually starts out as the precursor procollagen, but it’s cleaved to produce its final, active form.
Let’s review. In this lesson, we learned about Post-translational modifications or changes made to a protein or polypeptide after translation.
There are several different types of modifications, only some of which we discussed in this lesson. In particular, Phosphorylation is the addition of a phosphate group by a kinase. Dephosphorylation is the removal of a phosphate group by a phosphatase.
Ubiquitylation is the addition of a ubiquitin protein. Glycosylation is the addition of a carbohydrate to a protein, forming a glycoprotein. Cleavage is the cutting of a polypeptide. Lastly, it it’s worth emphasizing that these processes are specific. Only some proteins are targeted for these modifications, and these proteins are not always modified. In addition, specific enzymes target these proteins.
After completing this lesson, you should be able to:
- Understand what post-translational modifications of proteins does
- Identify the major modifications which can take place- like phosphorylation
- Recognize some of the minor modifications- like cleavage