Movement of Larger Molecules Across Membranes: Potocytosis

00:01 Okay, those are small things.

00:02 Those are individual ions or individual amino acids, or individual sugars were transporting.

00:08 Let's transport slightly larger things.

00:10 And there are three flavors of this, so we're transporting bigger things across the membrane.

00:15 We're gonna talk about potocytosis, which is cell sipping.

00:18 We're gonna talk about receptor-mediated endocytosis, also called pinocytosis, which is cell drinking.

00:25 And we're gonna talk about phagocytosis, which is cell eating for very big things.

00:32 So, we'll talk about each of these in turn. First up, potocytosis.

00:37 Potocytosis occurs at those lipid rafts.

00:41 Remember the lipid raft where we're talking about the asymmetry and the non-uniformity of the plasma membrane in a previous topic discussion? At those sites of the lipid raft where we have a lot of sphingomyelin, we also have a lot of a protein that likes to go to those lipid rafts called caveolin.

00:58 And that caveolin in the lipid rafts, on a regular constituent basis, invaginate to create a little invagination at caveola.

01:08 Literally, a little cave.

01:10 And that little cave then will pinch off with a sip of the outside world.

01:15 It will also pinch off with some other proteins and receptors that are -- that tend to localize to the lipid rafts and like to be next to caveolin.

01:25 So, the folate receptor, an important vitamin receptor, other GPI-linked proteins, and other signaling molecules for -- such as cyclic adenosine monophosphate binding proteins, will also be in our little caves as they invaginate.

01:40 Those little caves form an endocytic caveolar carrier with a little sip of the outside world, plus anything that was bound up to those membrane receptors.

01:50 And we will deliver those to the endosome where we can then release folate, for example, or we can have other molecules be subsumed into the endosome where they can be further degraded.

02:05 And we will constantly recycle the little endocytic caveolar carrier back up to the cell surface. This is going on all the time.

02:14 So, this is potocytosis. Literally, cell sipping.

02:19 Caveolae are involved in the cell sipping, as we've just said, and they will take in little bits of the outside world which will end up in the endosome, and the endosome can process them further.

02:30 It is a process that does not involve clathrin, as compared to what we're gonna talk about next, which is pinocytosis, and it occurs at lipid rafts. Caveolin is a structural protein and when it oligomerizes that it, say, associates with its fellow caveolins, that's what drives the process of invagination.

02:51 And not only it's a way to get in folate, but it's also a way to regulate a number of receptor molecules that we may want to down regulate.

03:01 So, for constantly taking them in, taking them to the endosome, endosome is processing them, we may be able to regulate membrane receptors in this manner.

03:12 So, it does a number of things besides sipping the outside world.