![]() Positively charged residues of membrane proteins have long been known to function as determinants of membrane topology, which is reflected in a statistical rule of membrane topology, i.e. We propose that positive charges are independent translocation regulators that are more active than previously believed. The arrest effect was prevented by negatively charged residues inserted into the positive-charge cluster, and it was also suppressed by high salt conditions. The translocation-arrested polypeptide was not anchored to the membrane and the charges were on the cytoplasmic side of the membrane. The C-terminus of the polypeptide chain was elongated during the arrest, and then the full-length polypeptide chain moved through the translocon. Here we show that positive charges temporarily arrested ongoing polypeptide chain movement through the ER translocon by electrostatic interaction, even in the absence of a hydrophobic segment. Positive charges on the nascent chain determine the orientation of the hydrophobic segment as it is inserted into the translocon and enhance the stop-translocation of translocating hydrophobic segments. As in the textbook description, adding chloride conductance makes the basolateral membrane less negative than if there was only a potassium conductance, because chloride moves out of the cell as the inside becomes more negative.Polypeptide chains synthesized by membrane-bound ribosomes are translocated through, and integrated into, the endoplasmic reticulum (ER) membrane by means of the protein translocation channel, the translocon. ![]() When you have multiple ion species involved, the equation to use is the Goldman equation which is related to the Nernst equation but involves a relative weighting of different ions by their concentrations and permeabilities. You can calculate this "reversal/equilibrium voltage" using the Nernst equation. Very few ions have to actually move to develop a biologically relevant potential (see explanations in other answers involving neurons: and ), so you can treat the ion concentrations as unchanged (and they will be, out to a fraction of a percentage point) despite this movement.Īt some voltage, the negative charge inside the cell will counteract the movement of potassium, so it's not like this is a constant deluge of potassium flowing out, it's more of a maintenance trickle. That flow causes a negative charge to develop inside, because the potassium diffusing out is positively charged. That means that, given an available path for potassium to diffuse across the membrane, there will be a net flow of potassium out of the cell. The other missing piece of information is that the outside space is (relatively) low in potassium. The explanation is in the caption text, quoting from your figure:īecause the apical membrane is conductive primarily to K+, the apical membrane voltage is more negative than the basolateral membrane voltage, which is conductive to K+ and Cl. Students of neuroscience are often confused by a very similar thing, as the resting potential of neurons (as well as voltage during action potentials, for that matter) is determined the same way. This is shown as the "yellow tubes" with dotted lines in the diagram. ![]() Don't pay attention to the stoichiometry of the pumps, that will only fool you! The charge is due to passive movement through open channels.
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