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.
-If DoRA aggregation is caused by charge neutralization, then aggregates should form as pH:

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.
-If DoRA aggregation is caused by charge neutralization, then aggregates should form as pH:

Quizplus · Accepted Answer

11ecba2d_0edb_720c_a3bf_29277ddfbb5a_MD0008_00 The isoelectric point (pI) of a protein is the pH at which the net charge of the protein is neutral. If the ambient pH is lower than a protein's pI, the protein gains protons and becomes positively charged. If the pH exceeds the pI, the protein loses protons and becomes negatively charged. The pI can be determined by isoelectric focusing, in which proteins migrate through a pH gradient until they become neutrally charged and are no longer influenced by an electric field. The pH at which a protein stops migrating is its pI. The scientists in the passage proposed that reflectins aggregate as their net charges are neutralized. If this is correct, then DoRA should have the greatest tendency to aggregate when the ambient pH is equal to its pI. Figure 2 shows that the pI of DoRA is approximately 9, so DoRA aggregates should form as the pH increases from 7 to 9. (Choice B) Decreasing the pH from 7 to 5 would move the ambient pH far below the pI of 9 and increase the positive charge on DoRA. As a result, the tendency of DoRA to aggregate would be greatly reduced. (Choices C and D) DoRA should have the greatest tendency to aggregate at a pH of 9, where it has a neutral charge. Moving the pH away from 9, either by increasing it or decreasing it, would reduce the tendency of DoRA to aggregate. Educational objective: The isoelectric point (pI) of a protein is the pH at which the protein is neutrally charged and does not migrate in an electric field. When the pH is lower than the pI, the protein becomes protonated and is positively charged. When the pH exceeds the pI, the protein loses protons and becomes negatively charged.

Passage Many Cephalopods Can Change Color to Blend in with Their