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.
-What effect does the addition of ACh have on the net charge of DoRA and DoRB in D. opalascens skin cells?

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.
-What effect does the addition of ACh have on the net charge of DoRA and DoRB in D. opalascens skin cells?

Quizplus · Accepted Answer

11ecba2d_0edd_1fbe_a3bf_a35cbe3e73da_MD0008_00 Post-translational modifications are additions of non-amino acid structures to proteins. They are frequently added and removed to modulate protein activity. One of the most common post-translational modifications is phosphorylation, in which a phosphate group is attached to a serine, threonine, or tyrosine residue. Phosphates are negatively charged, and phosphorylation causes a protein to become more negatively charged. In contrast, dephosphorylation makes a protein more positively charged. The passage states that in D. opalescens skin cells, ACh causes a signal cascade that results in phosphorylation of DoRA and dephosphorylation of DoRB. Therefore, in the presence of ACh, the net charge of DoRA becomes more negative as phosphate groups are added, and the net charge of DoRB becomes more positive (or less negative) as phosphate groups are removed. (Choice A) Only DoRB becomes more positive. DoRA becomes phosphorylated in the presence of Ach, and therefore becomes more negatively charged. (Choice B) This option would require DoRA to become dephosphorylated and DoRB to become phosphorylated in the presence of ACh. The passage states that the opposite mechanism is true. (Choice D) Only DoRA becomes more negative. DoRB becomes dephosphorylated in the presence of Ach, and therefore becomes more positively charged (or less negatively charged). Educational objective: Proteins are frequently modified post-translationally with non-amino acid groups such as phosphates. Phosphorylation results in more negative charges on a protein. Dephosphorylation makes a protein more positively charged.

Passage Many Cephalopods Can Change Color to Blend in with Their