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BBio 351: Principles of Anatomy & Physiology I
Winter 2016
Study Guide for Maude W. Baldwin et al. (2014)
Note: This is not a study guide in the sense of “here are some questions to prepare you for the
test.” It is a study guide in the sense of “here is some background information to help you read
the paper.”
Reference: Maude W. Baldwin et al. (2014), “Evolution of sweet taste perception in
hummingbirds by transformation of the ancestral umami receptors,” Science 345: 929-33.
Table of contents
1. The bottom line
2. A helpful summary
3. Brief background
4. A few notes on methodology
5. Homework assignment
1. The bottom line
The overall conclusion of this study is that hummingbirds are able to taste sweet things (like
nectar) thanks to multiple mutations that transformed their umami receptors into receptors
that can recognize sugars (rather than amino acids). As with the previous paper, our big
challenge is to understand how the data shown in the figures lead to this overall conclusion.
2. A helpful summary
The issue of Science that included Baldwin et al.’s paper also included a short, less technical
summary by Peihua Jiang and Gary K. Beauchamp (“Sensing nectar’s sweetness,” Science 345:
878-9, 2014). I recommend that you read it as a warm-up for reading the Baldwin article.
3. Brief background
There are five known taste receptors, which sense
sweet, bitter, umami (“savory”), acids, and salt,
respectively. Sherwood Figure 6-23 shows this
somewhat confusingly – it appears that sweet,
bitter, and umami are all sensed by the same
receptor, which is not quite true. A more realistic
view of these receptors is shown in the figure below
(from Jayaram Chandrashekar et al.,
Nature 444, 288-94, 2006). Note that the receptors
for sweet and umami are both heterodimers (2
different protein subunits).
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BBio 351: Principles of Anatomy & Physiology I
Winter 2016
4. A few notes on methodology
On the whole, I found this paper easier to read than the previous one. A few less
straightforward aspects are highlighted here….
 Phylogenetic trees (Figure 1). I will assume that you have a conceptual understanding of
how phylogenetic trees group genetic sequences according to relatedness. Note that,
according to Figure 1, T1R3, T1R2, and T1R1 are all derived from an ancestral T1R gene.
 Receptor activity assays (Figures 2 and 3). These assays were not described fully in the
paper or its online supplement, so here is a quick explanation. The components of the
receptor proteins (e.g., T1R2 and T1R3), are expressed inside embryonic kidney cells
(Hek293t), which then respond to taste stimuli as normal taste receptors would,
triggering a rise in intracellular calcium. This calcium signal can then be made visible via
a calcium-dependent photoprotein, which emits light (luminescence). Thus, the Y axis
of Figures 2A, 2B, 2C, 3A, and 3B show the amount of luminescence detected, indicating
the extent to which a particular stimulus was received.
 Homology modeling. Solving the 3-dimensional structure of a new protein can help
elucidate its function, but is often difficult to do. When a protein’s 3D structure cannot
be solved empirically, a “homology model” is often constructed, based on the solved
structure of another similar protein. By aligning the amino acid sequences of the two
proteins, the approximate shape of the new protein can be deduced.
 Behavioral assay. The experiments shown in Figure 4 offered hummingbirds a choice of
two stimuli to see which one they would prefer. The pairs of gray bars may be
confusing; what was the choice in those cases? The online supplementary materials
explain: “In between trials, birds were given sucrose in both cuvettes to prevent a side
bias from developing, and to ensure that an interest in feeding persisted.”
4. Homework assignment
Consult Canvas for the homework assignment that is due Wednesday, March 9th by the start of
class.
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