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Olfactory receptors

Animals sense their chemical environment through olfactory receptors (ORs). The olfactory receptors are a large group of proteins belonging to a subfamily of G protein-coupled receptors (GPCRs) that bind odorant ligands. If the receptor is activated by a ligand, the confirmation of the receptor is changed and there is an interaction with the α-subunit of the G-protein. This causes dissociation of the α-subunit from the Gβγ dimer and the signal is propagated [1]. Because of the sensitivity and selectivity of the of the olfactory system it can be of value in detection of environmental toxins [2] or pharmaceutical screening. In this iGEM project we aim to investigate if the ORs can be used as a diagnostics tool for tuberculosis.

Yeast G protein-coupled receptors

In this project we choose to work with the budding yeast Saccharomyces cerevisiae as a host organism because it utilizes already a GPCR pathway. Furthermore S. cerevisiae has been successfully used for functional expression of GPCR’s [3,4], a lot of genomic tools are available, and it has a fully characterized genome. In S. cerevisiae two GPCR cascades have been identified: a glucose sensing pathway and a mating pathway [5]. There are two sexes of yeast cells, MATa and MATα. Whenever pheromones (small peptides) of the opposite sex are bound to the specific G-protein coupled receptors (Ste2 p or Ste3p), the MAP kinase cascade is turned on, leading to induction of mating genes such as FUS1 and growth arrest mediated by the FAR1 promoter. This mating response can be seen in the form of a morphological change, called shmoo formation. In figure 1 an overview of the pheromone and glucose signaling pathways in S. cerevisiae is shown.

Overview of pheromone and glucose signaling in S. cerevisiae. Figure adapted from Versele et al.

Introduction of a new olfactory receptor

Previously it was found that that the yeast pheromone signaling pathway can be coupled to a mammalian olfactory receptor. Minic et al. succeeded in functional expressing the rat 17 OR and its trafficking to the plasma membrane in S. cerevisiae. Between the three GPCRs that are known in S. cerevisiae, Ste2, Ste3 and Gpr1, the sequence similarity is limited. Except for pheromone receptors in Schizosaccharomyces pombe and Kluyveromyces lactis, Ste2 and Ste3 are largely unrelated in sequence to other GPCRs [5]. Nevertheless, the yeast pheromone receptors can be functionally replaced by several mammalian GPCRs so that the pheromone pathway can be activated by the corresponding ligands [4].

Chimeric design

A major hindrance for functional expression of ORs has been that the receptors did not localize in the membrane or that the downstream coupling of the receptor to the Gα did not work properly. It has been shown that the rat olfactory receptor 17 (R17) that responds to octanal can be functionally expressed in many different cell types, including S. cerevisiae [6]. Earlier research investigated on the question whether the RI7 sequence can be used to functionally express other ORs. Sequence analysis of ORs have shown that the N-termini of the receptor are involved in plasma membrane localization, whereas the C-termini generally define the specificity for G protein interaction [7]. Based on this observations Radhika et al. functionally expressed a chimeric OR with the N-terminus and the C-terminus of the RI7 sequence. A schematic picture is shown in figure 2. In this iGEM project we use the same approach as Radhika et al. by substituting the receptor termini with the RI7 sequences.

Schematic overview of the chimeric design of the receptor. Figure adapted from Radhika et al..

Niacin olfactory receptor

The receptors GPR109A and GPR109B are known to bind the compound nicotinic acid [8]. It was previously described that GPR109B acts a low affinity receptor for nicotinic acid and GPR109A acts as a high affinity receptor for nicotinic acid and other compounds with related pharmacology [molecular identification of high and low affinity receptors]. The chemical compound methyl nicotinate is closely related to nicotinic acid. Because one of the compounds in the breath of tuberculosis patients is methyl nicotinate [9,10], the high affinity receptor for niacin is a good candidate for testing the ‘olfactory yeast’ as a diagnostics tool.

Isoamylacetate olfactory receptor

The first iGEM team of MIT 2006 made a biobrick called the ‘banana odor generator’. With this part E. coli cells can generate the isoamyl acetate molecule. We aim to let yeast detect this isoamyl acetate signal with a olfactory receptor. The idea is that in future work the yeast should couple this back to the bacteria to have gaseous yeast/bacteria communication on plates.
The human receptor OR1G1 and mouse receptor Olfr154 are known to react on isoamyl acetate [11] and therefor these two receptors were used in this iGEM project.


A design of the receptor construct was made with the olfactory receptors placed between the N-terminal and the C-terminal part of the rat I7 receptor. As a promoter the strong constitutive GPD promoter is used and as a terminator the CYC1 terminator. The receptor can be replaced by using the restriction sites BamHI and NdeI. A FLAG tag is added upstream of the receptor sequence to look at the localization of the receptor in the membrane. The plasmid construct for the receptor expression was obtained by restriction of the synthesized receptor construct and ligation in the pRSII415 expression vector. The following biobricks are created:






After transformation of the plasmids in the yeast strain a PCR reaction was performed in order to verify if the plasmid was correctly transformed. Since the PCR reactions were performed with single colonies we expected to obtain one PCR product with the length of the receptor part. However, for all the receptors we saw multiple PCR products on the gel; products with the length of the receptor, and products indicating that only the plasmid backbone was present (without the receptor) . This can indicate that during growth of the yeast a part of the plasmid was emitted.

Expression and localization of the ORs


I7-GPR109A transformed cells and WT cells were stained with Conjugated anti-FLAG antibodies according to the immunofluorescence staining protocol and viewed under a widefield fluorescence microscope with the goal of imaging the expression of our GPCR chimeras and image their localization in the cell. The image was analyzed with Image J to compare the fluorescence of the cell and cell membrane to the overall fluorescence of the whole picture.



It can be seen that there is expression of the receptor: I7-GPR109A transformed cells are fluorescent (figure 4) and the wild type strain is very weakly fluorescent (figure 5). In some of the I7-GPR109A transformed cells there is clear halo structure visible, which indicates localization of the receptor on the membrane. Below such a typical halo is shown (figure 4):

Figure 4: The I7-GPR109A transformed cells


Figure 5: Wild type (WT)

In figure 4, it can be seen that localization to the cell membrane occurs. This differs substantially from the control (figure 5, right picture). For the above cell, the mean of the gated fluorescence was a value of 1592.1 compared to a fluorescence mean of 1245.2 for the total picture, yielding a ratio of 1.28 for the I7-GPR109A strain. For the WT the cell mean fluorescence value was 4855.9 for the mean background and 4942.9 for the selected sample typical cell, yielding a ratio of 1.018 for the sample, indicating little fluorescence compared to background. Below a summary of the values:




Mean cell fluorescence

Mean total fluorescence

Fluorescence Ratio

I7-GPR109A transformed cells




Wild type Strain (WT)






The absolute difference between the values of the wild type and the I7-GPR109A photos (WT strain having a much higher mean cell fluorescence) is the result of our efforts to gain a visual result from the weak fluorescence of the WT strain by increasing the time for imaging. So although the absolute fluorescence is higher in the WT the ratio still indicates expression for the I7-GPR109A transformed cells


Figure  6: Localization analysis of I7-GPR109A transformed cells.

In figure 6 the graph of the yellow pixel line indicates a higher intensity (intensity slice shown on the right) at the sides of the cell and therefore confirms the expected protein localization pattern.





Ligand activation


If the downstream pathway of the olfactory system is activated one of the responses is that the cell goes in growth arrest. If the cells go in growth arrest they will stop growing in the G1 phase and hence the DNA content of the cells should be 1N. By staining the DNA of the cells with a fluorescent dye we watched with flow cytometry at the DNA content and thereby at the cell cycle phase.
The niacin receptors and the two isoamylacetate receptors were induced with the ligand. After staining (see protocols) the DNA content was measured.

Flow cytometer results of I7-Gpr109A transformed cells induced with ligands. Cells were DNA stained and measured after 4.40 hours.

The picture shows DNA content distribution of two strains, wildtype cells and cells transformed with I7-Gpr109A, 4.40 hours after induction. The I7-Gpr109A without induction shows one peak. WT cells with nicotinic acid show similar cell clouds and peak intensity. The alpha pheromone induced cells however shows a small cloud shifting towards the left. The methyl nicotinate induced cells shows a peak similar to the non-induced cells. The I7-GPR109A transformed cells however, show two clear clouds after 4.40 induction. This indicates that DNA replication has halted, leaving the cells in their haploïd state.


An olfactory receptor placed between the N-terminal and the C-terminal part of the rat I7 receptor is expressed in yeast. In some of the cells there is a halo structure visible, this indicates localization of the receptor on the membrane.


During growth of yeast cells transformed with the expression vector we observed two things: not all the cells maintain the right plasmid and the cells grew slower than wild type cells. A reason for this could be that the expression of the receptor is disadvantageously for the cells. Therefor we recommend for future work to use an inducible promoter instead of a strong constitutive promoter. In that case one can make the yeast cells expressing the receptor just before testing the strain.


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[2] Venkat Radhika, Tassula Proikas-Cezanne, Muralidharan Jayaraman, Djamila Onesime, Ji Hee Ha &Danny N Dhanasekaran, Chemical sensing of DNT by engineered olfactory yeast strain Nature Chemical Biology 3 (2007)
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