[SOUND] Welcome. We're going to continue our exploration of a little more detail of the synthesis and secretion of some of the milk components. So in this video, we're going to focus mostly on the protein synthesis and secretion. Particularly in milk proteins, remember the major milk proteins are the caseins and the whey proteins. And remember, we're also recall that the caseins are kind of packaged in this thing called a casein micelle. And we'll address a little bit of that here in a few moments. We're just going to really hit superficially protein synthesis, but just to kind of remind you of how this happens in the mammary cells. So let's go to the first slide. And to remind you what the cell looks like, mammary epithelial cell. Here's the nucleus here, the basal membrane would be down here. So basically this is a cell kind of like this. The apical membrane up here, a fat droplet about ready to be secreted off or released from the cell. Secretory vesicles full of casein micelles. These little black dots here, the really black dots, are the casein micelles that I was talking about. And you can't really see the Golgi apparatus, that'll come into play. And in this magnification you really can't see the rough endoplasmic reticulum, which is another part that's important to us. So, again, what I've done here is to make a diagram, so let's go to the next slide. To kind of diagram out these key organelles that are involved in protein synthesis of the milk proteins. So again, we have a cell, we have a basal lateral membrane going up to a tight junction. And remember, those epithelial cells kind of form a physiological membrane. That is to say, this side of the cells are different from this side of the cells and that's because of this tight junction. So there's tight junction's between the two cells. There's a cell over here, the one we're going to look at, and then I'm standing in the middle of a cell right here. Nucleus, cytosol, Golgi apparatus, again, remember that's where lactose is being produced. It's also where proteins are being kind of modified and processed. These secretory vesicles, membrane bound secretory vesicles, they butt off of the Golgi apparatus, work their way up to the apical membrane, and then release their contents out into the lumen. We also have then, to remind you of these, rough endoplasmic reticulum. So rough endoplasmic reticulum, there's a lumen, I've just drawn a line here, and then the little circles here represent the ribosomes. So ribosomes, those polysomes, they're attached to the outer, or the cytosolic side of the membrane for the Golgi, excuse me, the rough endoplasmic reticulum. The ribosomes translate the messenger RNA, the polymerised amino acid string there, which becomes protein, is inside the rough endoplasmic reticulum. Those proteins are then, by little microvesicles, transferred over to Golgi apparatus where all sorts of things start to happen to them. And we'll get into a little bit of that here in just a moment. Go to the next slide. Just to kind of remind you again. This is a blown-up picture of an electron micograph of rough endoplasmic reticulum. It's kind of showing the studded, the darker things here, the darker dots or the ribosomes. Again, they're on the cytosolic side of the membrane. Inside that membrane would be where the nascent, the newly polymerised proteins end up. And then, again, they're transferred over to the Golgi apparatus. So let's start looking at how this happens with regard to protein synthesis. Amino acids, all the amino acids are coming from the blood. Are transported across the membrane of the basal lateral membrane in the epithelial cell. There are transporters, very specific transporters. So they will transport certain groups or types of amino acids. For example, branched-chain amino acids have a particular kind of transporter. Other kinds of amino acids will have a different kind of transporter. So there's a variety of different transporters that transport amino acids into the cell, into the cytosol. Those then are polymerised, again, at the point of the RER, again, into these long amino acid chains we call proteins. And those are in the lumen of the RER. Those are then transferred by these small vesicles over to the Golgi apparatus. That's the point at which they would become glycosylated, phosphorylated. Remember, a lot of the caseins are phosphorylated. Some of the caseins, kappa-casein, for example, is glycosylated. Many of the whey proteins are glycosylated. So that's all happening in the Golgi apparatus. Some of that continues to happen then in the secretory vesicles as it moves up. This is also the point in the Golgi apparatus and continuing in the secretory vesicles of where the casein micelles start to form. So all those casein molecules, the calcium, the phosphorylated casein and phosphate pull together, as we've talked about in another video, forming that casein micelle. So that's these white dots here, represented those. And again, they're in the secretory vesicles. As these things kind of move their way up to the apical membrane. The casein micelles start to kind of condense a little bit, they become more and more electron-dense. And so a lot of the ones that we see in the lumen out here are pretty electron-dense. Go to the next slide. So again, the casein micelles out in the lumen, secretory vesicles. We also find those being started to be formed in Golgi apparatus. Let's go to the next slide. So what I've done here is just to take an electron micrograph of a bunch of casein micelles that we'd find in the lumen. And again, these very dense, electron-dense dots here is what the casein micelles would look like. Now the other thing, if you noticed, if you were watching very carefully, is I've shifted these secretory vesicles up here so they're kind of lined up. So one of the interesting things that occurs in terms of secretion or release of these milk proteins, as well as the lactose, and a lot of the water is that these things tend to fuse together, so that they're not all individually going to the apical membrane and releasing the contents. They kind of line up and fuse. So let's go to the next slide. And you can see that here. So there's a bunch of secretory vesicles. Here's one here, there's another one here. Another one here, another one here, another one here, another one there. And if you look carefully, you start to realize these are kind of fusing together. These two have fused together here. These two have fused together. And so, as they hit that apical membrane, it's not just one at a time. They're not taking a turn one at a time, in essence. They seem to be kind of clustering together. So that by the time that the apical membrane and the secretory vesicle membrane fuse and open up, there's a bunch of the contents being released simultaneously. So this is kind of an interesting observation in these cells. So let's do a real quick review here. Again, amino acids are taken up by the specific transporters across the basal lateral membrane. They are then polymerised at the point of the rough endoplasmic reticulum. They're in turn transferred to the Golgi apparatus, again, glycosolated. Again, a variety of different proteins, especially whey proteins, but also kappa-casein are glycosolated. The casein micelles start to be formed in the Golgi apparatus. And then continue, these processes continue in terms of the secretary vesicles until they're finally released into the milk. There is one exception to this. Actually, a couple of exceptions in terms of milk protein. One of them, and we'll get to it briefly in another video in terms of milk fat synthesis, are the proteins that surround the milk fat globule inside the cell. And they're produced in ribosomes out in the cytosol. They're not a secretory type of protein in that sense. The other type are immunoglobulins, and some other proteins. Are actually taken up by small vesicles at the basal lateral membrane, transported through the cell, and released out into the lumen. So immunoglobulins, for example, do not enter this pathway that we've just talked about here. They're already preformed in the blood, and then they're taken up by these vesicles, usually receptor mediated. There's some sort of receptor involved, so some specificity to that uptake, and then transported across, released out into the lumen. So again, several different mechanisms by which proteins are secreted from the mammary gland. Milk-specific proteins, though, are going to be going through this route that we just talked about. 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