Hey people! I’m here with Inspector Bunny, who is well known for annoying ducks with all his constant digging of rabbit holes. Inspector Bunny, would you mind describing your latest rabbit hole?
No problem. We began with a core tension that is built into the fabric of life itself. The themes of Compartmentalization and Adaptability are universal in life. Compartmentalization is needed to sequester the machinery of life from the environment, but this machinery cannot be completely cut-off, otherwise it would not be able to help life adapt to the challenges posed by a changing environment. The perfect solution to this dilemma is the membrane composed of lipids and proteins. The lipid bilayer serves the needs of Compartmentalization while the embedded protein sensors, channels, and pumps serve the needs of Adaptability. Thus, the membrane, composed of lipids and proteins, is universal in life.
So are you saying the membrane must have been designed? Because, like, it’s so darn complex??
Silly reporter. No, don’t get side-tracked by such questions. The truly remarkable image comes from focusing on the elegance of the system, not from what must have happened or could not have happened.
Okay, so focus on the elegance of the system.
The membrane itself is truly elegant, both structurally and conceptually, but it itself comes with its own built in tension. All proteins are stitched together, one amino acid at a time, by the nanotech machines known as ribosomes. When the newly formed proteins emerge from the exit tunnel of the ribosome, they fold into conformations that are essential for their function. But because the cytoplasm of the cell is mostly water, the proteins fold in such a way that the oily amino acids are mostly buried in the central core of each protein, forming the conformational back-bone of the protein. The surface of the proteins is decorated with amino acids that can interact with the surrounding water molecules.
Well, this poses a very serious problem for the proteins embedded in the membrane, or any protein trying to get across a membrane. The lipid bilayer is a very different environment than the watery cytoplasm and repels those dissolved, folded proteins whose surface amino acids are complexed with water. That’s why it so perfectly satisfies the needs of Compartmentalization, as anything, including proteins, dissolved in water can’t get through the lipid bilayer. So how do you get protein sensors or channels in the membrane when the membrane is supposed to keep proteins, among other things, from getting across it!? You simply take the proteins that are supposed to be in the membrane and turn them inside out, so the oily residues are on the surface and find themselves quite at home in the oily layer of membranous lipid.
Okay, so how do you turn a protein inside out?
The structure of any protein is determined by its amino acid sequence. So all you would need to do is string together various amino acids in a sequence that would cause the protein to fold inside-out….if it was in an oily environment. And there’s the catch.
Oh, I think I get it now. The ribosome does not make proteins inside membranes. It exists inside the watery cytoplasm. So if the ribosome made membrane proteins inside the cell, the proteins could not fold properly.
Yes, and what’s worse, these unfolded membrane proteins would end up existing as gobs of oily goo. And when goo bumps into other goo, it becomes a growing goo that will gunk up the whole cell.
Sounds icky. So what keeps the goo away?
That’s where the signal recognition particle hops in! The signal recognition particle, or SRP, extends the theme of elegance as it…..
Hold that hare-brained thought Inspector, as we need to take a commercial break. Don’t go away folks, when we come back, the Inspector will tell us all about this SRP thingy.