DRAFT
The Making of the Fittest: Natural Selection and Adaptation
Biochemistry and cell signaling pathway of the Mc1r gene
Overview:
In this classroom activity, students use amino acid sequences from the rock pocket mouse
genome to illustrate the different levels of protein structure and the relationship between a
protein’s structure and function. In addition, students analyze the MC1R pathway in both light
and dark-colored rock pocket mouse populations. Through their analysis, students will come to
understand the concepts of cell signaling. Finally, students hypothesize how the change in
sequence and thus protein function might directly affect the coat-color cell signaling pathway of
the rock pocket mouse populations.
Objectives:
The students will be able to:

use an amino acid chart to determine the class of each amino acid (nonpolar, polar,
acidic (-), basic (+)) in a protein sequence.

understand that protein functions are determined by the structure of the protein, which is
governed by the amino acid sequence.

comprehend the concept of protein folding into various levels of protein structure.

understand the concept of cell signaling, including reception, transduction, and response.

understand the concept of the Mc1r pathway as it relates to coat-color in rock pocket mouse
populations.

analyze the amino acid data to hypothesize how the change in sequence of the protein
domains might affect the functionality of the protein.

analyze the amino acid data to hypothesize how the change in sequence and thus protein
function might directly affect the coat-color of the rock pocket mouse populations.
Key Words:
amino acids, protein, protein structure, cell signaling, receptors, transduction pathways
Time Requirement:
one 50-minute class period; additional time for the analysis questions might be required
depending on pace of students
Appropriate Levels:
high school biology (AP, IB), introductory college biology
Prior Knowledge:
Students should understand the relationship between protein structure and function and should
be comfortable with the processes of transcription and translation. Students should also be
familiar with the basics of cell signaling, including the function of ligands, reception, transduction
pathways, second messengers, and response.
Materials:

amino acid class chart

blue, red, green, and yellow colored pencils
HHMI
Biochemistry and cell signaling pathway of the Mc1r gene: Teacher materials
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The Making of the Fittest: Natural Selection and Adaptation
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Activity Tips:



Although this activity can be completed as a standalone activity, we recommend that the
activity Molecular genetics of color mutations in rock pocket mice be completed first.
Students can complete the procedure individually or in small groups. Analysis questions are
designed to be challenging and should be completed in student groups of 3 or 4 students to
encourage discussion and help to identify the misconceptions.
Students use color to classify amino acids types in Step 3 of the procedure. If you do not
have access to a color printer, you should compare student work to the answer key (page 2
of this document) on your computer screen.
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Biochemistry and cell signaling pathway of the Mc1r gene: Teacher materials
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DRAFT
The Making of the Fittest: Natural Selection and Adaptation
Answer Key:
Wild-type Mc1r gene (light phenotype)
015
Amino
Acid
Asn
Extracellular Domain I
Ser
Thr
105
Amino
Acid
Ala
Arg
Thr
Gly
024
Val
Pro
His
Extracellular Domain III
Leu
Ala
Thr
Arg
Val
Leu
114
Thr
Val
Val
Gln
Transmembrane
Domains
154
Amino
Acid
Ser
Intracellular Domain I
Ile
Val
230
Amino
Acid
Leu
Thr
Leu
Pro
163
Arg
Ala
Arg
Intracellular Domain III
Val
His
Gln
Gly
Phe
Trp
239
Arg
Leu
Lys
Gly
Mutant Mc1r gene (dark phenotype)
015
Amino
Acid
Asn
Extracellular Domain I
Ser
Thr
105
Amino
Acid
Ala
Cys
Thr
Gly
024
Val
Pro
His
Extracellular Domain III
Leu
Ala
Thr
Trp
Val
Leu
114
Thr
Val
Val
Gln
Transmembrane
Domains
154
Amino
Acid
Ser
Intracellular Domain I
Ile
Val
230
Amino
Acid
HHMI
Leu
Thr
Leu
Pro
163
Trp
Ala
Arg
Intracellular Domain III
Val
His
His
Gly
Phe
Trp
239
Arg
Leu
Biochemistry and cell signaling pathway of the Mc1r gene: Teacher materials
Lys
Gly
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DRAFT
The Making of the Fittest: Natural Selection and Adaptation
Questions:
1. The melanocortin-1-receptor is embedded in cell membranes of specialized cells called
melanocytes. It plays a role in the determination of coat-color.
2.
a. These shapes represent the phospholipid molecule found in all cell membranes. They
make up the phospholipid bilayer structure of membranes.
b. They are present to represent the membrane in which the receptor is embedded. In
addition, the MC1R protein is a transmembrane protein with extracellular domains,
transmembrane domains, and intracellular domains. Inclusion of the shapes on the data
page helps to visualize the portion of the protein extending out of the cell (extracellular)
and the portions extending into the cytoplasm (intracellular).
3. Sample flowchart:
MC1R receptor
(G proteincoupled)
α-MSH hormone
ligand
cAMP
(secondary
messenger)
G protein
Eumelanin
production
(dark color)
4. Completed table
Amino Acid Mutation
Position Number
Example 1
018
109
160
233
Wild-type Mc1r
Amino Acid Chemistry
Polar (hydrophilic),
neutrally-charged
Electrically-charged,
positive (basic)
Electrically-charged,
positive (basic)
Electrically-charged,
positive (basic)
Polar (hydrophilic),
neutrally-charged
Mutated Mc1r
Amino Acid Chemistry
Electrically-charged,
negative (acidic)
Polar (hydrophilic),
neutrally-charged
Nonpolar (hydrophobic),
neutrally-charged
Nonpolar (hydrophobic),
neutrally-charged
Electrically-charged,
negative (acidic)
5.
a. The extracellular mutations at positions 018 and 109 both change a positively-charged
arginine amino acid to a neutrally charged amino acid (polar cysteine and nonpolar
tryptophan, respectfully). This change in electrical charge in the extracellular domains of
the receptor protein could increase the affinity of the ligand for the receptor therefore
amplifying the pathway signal thus producing more eumelanin. The change in charge
could also decrease the effect of an antagonist from another gene which would also lead
to more eumelanin produced and a dark coat-color.
b. The missense mutation found at position 160 changes a positively charged arginine to a
neutral, nonpolar tryptophan, and the mutation at position 233 replaces glutamine which
is neutral and polar to a positively charged and basic histidine. These changes will
ultimately change the shape of the intracellular domains of the protein perhaps
producing an overly active receptor which would lead to the production of eumelanin. In
HHMI
Biochemistry and cell signaling pathway of the Mc1r gene: Teacher materials
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The Making of the Fittest: Natural Selection and Adaptation
DRAFT
addition, this change could increase the activation of the G protein without the need of a
ligand. This increase in activation would amplify the levels of cAMP, thus increasing
eumelanin production.
c. The normal receptor conformation requires a ligand for activation of the G protein. If the
ligand is not readily available, the production of eumelanin will decrease resulting in a
light coat-color. Another hypothesis might be the presence of an antagonist from another
gene in the rock pocket mouse which when expressed interferes with the MC1R
pathway thus reducing the eumelanin production.
Note to teachers regarding Question 5: It has been shown that there is a second gene
involved in this pathway namely the agouti gene. This gene produces a protein antagonist to the
MC1R pathway. The presence of this antagonist decreases the production of eumelanin and
increases the production of pheomelanin. Obviously, the light coat-colored mice contain the
normal MC1R receptor and related pathway, and they also contain the agouti gene. During the
hair development cycle, there is a pulse of agouti expression producing the antagonist that
results in the deposition of pheomelanin in the hair. This results in the light coat-color
phenotype. In the mutated mouse MC1R receptor research, preliminary results indicate that the
mice with the dark alleles have hyperactive receptors which increase the levels of cAMP, thus
producing eumelanin in the melanocytes resulting in the dark phenotype, even though these
mice also contain the agouti gene and the protein antagonist.
Author:
Ann Brokaw
AP Biology Teacher
Rocky River High School
Rocky River, Ohio
HHMI
Biochemistry and cell signaling pathway of the Mc1r gene: Teacher materials
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