Passage
Many cephalopods can change color to blend in with their surroundings by altering the conformations of particular proteins within their skin cells. The reflectin proteins play a major role in this camouflage mechanism. Reflectins are a group of intrinsically disordered proteins that do not adopt clearly defined secondary or tertiary structures. Under certain conditions reflectins aggregate, allowing cells to reflect various wavelengths of light.Skin cells from the squid Doryteuthis opalescens were cultured and three reflectins-DoRA, DoRB, and DoRC-were purified and analyzed to determine amino acid composition. They were found to contain an abundance of arginine and serine but relatively few cysteine, leucine, isoleucine, valine, phenylalanine, or tryptophan residues. Figure 1 shows the relative hydrophobicity of each reflectin variant as a function of amino acid position.
Figure 1 Hydrophobicity plots of three reflectins isolated from D. opalescens. (Note: Negative numbers indicate hydrophilic regions.) In D. opalescens skin cells (pH ≈ 7.8) , reflectins aggregate in response to acetylcholine (ACh) , which causes a signal cascade that changes the phosphorylation state of the proteins. Cells were incubated with or without ACh, and DoRA and DoRB were purified and assessed for phosphorylation by mass spectrometry. DoRA became phosphorylated in the presence of ACh whereas DoRB became dephosphorylated. After examining the isoelectric points of unmodified DoRA and DoRB, as shown in Figure 2, scientists proposed that reflectins aggregate upon charge neutralization.
Figure 2 Isoelectric focusing of DoRA and DoRB
Adapted from Demartini DG, Izumi M, Weaver AT, Pandolfi E, Morse DE. Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells. J Biol Chem. 2015;290(24) :15238-49.
-To generate Figure 2, reflectins were loaded onto the low-pH side of a gel with a pH gradient, and a current was applied. This resulted in:

Question

Passage
Many cephalopods can change color to blend in with their surroundings by altering the conformations of particular proteins within their skin cells. The reflectin proteins play a major role in this camouflage mechanism. Reflectins are a group of intrinsically disordered proteins that do not adopt clearly defined secondary or tertiary structures. Under certain conditions reflectins aggregate, allowing cells to reflect various wavelengths of light.Skin cells from the squid Doryteuthis opalescens were cultured and three reflectins-DoRA, DoRB, and DoRC-were purified and analyzed to determine amino acid composition. They were found to contain an abundance of arginine and serine but relatively few cysteine, leucine, isoleucine, valine, phenylalanine, or tryptophan residues. Figure 1 shows the relative hydrophobicity of each reflectin variant as a function of amino acid position.

Figure 1 Hydrophobicity plots of three reflectins isolated from D. opalescens. (Note: Negative numbers indicate hydrophilic regions.) In D. opalescens skin cells (pH ≈ 7.8) , reflectins aggregate in response to acetylcholine (ACh) , which causes a signal cascade that changes the phosphorylation state of the proteins. Cells were incubated with or without ACh, and DoRA and DoRB were purified and assessed for phosphorylation by mass spectrometry. DoRA became phosphorylated in the presence of ACh whereas DoRB became dephosphorylated. After examining the isoelectric points of unmodified DoRA and DoRB, as shown in Figure 2, scientists proposed that reflectins aggregate upon charge neutralization.

Figure 2 Isoelectric focusing of DoRA and DoRB
Adapted from Demartini DG, Izumi M, Weaver AT, Pandolfi E, Morse DE. Structures, Organization, and Function of Reflectin Proteins in Dynamically Tunable Reflective Cells. J Biol Chem. 2015;290(24) :15238-49.
-To generate Figure 2, reflectins were loaded onto the low-pH side of a gel with a pH gradient, and a current was applied. This resulted in:

Quizplus · Accepted Answer

11ecba2d_0ed9_763a_a3bf_4710da0f3a17_MD0008_00 In isoelectric focusing, an electric field causes proteins to migrate through a pH gradient within a gel. The gel is placed with the low-pH end near the anode (attracts anions) and the high-pH end near the cathode (attracts cations). At low pH, proteins are fully protonated, making them positively charged and causing them to migrate away from the anode and toward the cathode. Proteins that start at the high-pH end are initially deprotonated and negatively charged, so they migrate toward the anode. As positively charged proteins move toward the cathode, the pH to which they are exposed gradually increases, causing the proteins to become deprotonated and lose positive charge. When the net charge becomes zero, the protein stops migrating through the gel. The pH at which this occurs is the protein's isoelectric point. The question states that reflectins entered the gel at the low-pH end, which is near the anode. They are fully protonated and positively charged at this end, and they gradually become deprotonated as they migrate from anode to cathode. (Choice B) The low-pH end of the gel is near the anode, and proteins that enter at this end migrate toward the cathode. (Choice C) Reflectins migrate from anode to cathode, but become deprotonated as they do so. (Choice D) Reflectins would become protonated as they migrate from cathode to anode if they entered the gel at the high-pH side, not the low-pH side. Educational objective: In isoelectric focusing, an electric field causes proteins to migrate through a pH gradient. The low-pH end of the gradient is placed near the anode and the high-pH end is placed near the cathode. As proteins migrate from low pH to high pH (from anode to cathode), they lose protons and become less positively charged, stopping when their net charge becomes zero. The opposite is true of proteins migrating from high pH to low pH.

Passage Many Cephalopods Can Change Color to Blend in with Their