Phenotyping with phenylthiocarbamide (ptc) taster strips

Enable participants to use the polymerase chain reaction (PCR) and DNA sequence analyses to discover something new about themselves or the surrounding microbial environment

• Enable participants to use the polymerase chain reaction (PCR) and DNA sequence analyses to discover something new about themselves or the surrounding microbial environment

• Run 2-3 different projects, to keep group number to a manageable size and so different groups can present to and learn from each other

• Select genes for study that have easily-identifiable differences in DNA sequence in the test population, so that comparisons can be made between test subjects

• For projects using human subjects, select genes that are associated with interesting human traits

• Avoid selecting genes that are associated with human diseases/disorders or that may raise questions about paternity

• The time frame of the projects, from introduction to final presentations, must fit within a 10-12 day window for summer program, or in several 2-4 hour lab periods for a course

Phenotyping with phenylthiocarbamide (PTC) taster strips

• Phenotyping with phenylthiocarbamide (PTC) taster strips

• Background on the molecular genetics of the PTC taste receptor

• Analyze DNA sequence that “matches” your phenotype - can you find the polymorphisms?

• Location of the amino acid polymorphisms within the 3-D structure of the PTC taste receptor

The inability to taste certain compounds is usually due to simple, recessive Mendelian inheritance.

• The inability to taste certain compounds is usually due to simple, recessive Mendelian inheritance.

• Dozens of taste and odorant receptors have been cloned and sequenced in the last 20 years.

• The TAS2R28 gene encodes a bitter taste receptor that enables humans to taste the compound PTC.

• The PTC (TAS2R28) gene has a single coding exon, for a polypeptide chain with 333 amino acids.

Three common single nucleotide polymorphisms (SNPs) are associated with PTC sensitivity.

• Three common single nucleotide polymorphisms (SNPs) are associated with PTC sensitivity.

• Each SNP results in a change to the amino acid sequence of the PTC receptor.

What is the normal structure and function of the PTC receptor?

• What is the normal structure and function of the PTC receptor?

• Where within the structure do the non-taster variant amino acids reside?

• What is the side chain structure of the taster (PAV) and non-taster amino acids (AVI)?

• How do the variant amino acids alter the structure and/or function of the protein?

‘VI’ mutant variations shown as gray shadows

• ‘VI’ mutant variations shown as gray shadows

• Floriano et al. J Mol Model, (2006) 12: 931-941

The extracellular PTC binding sites between taster and non-taster receptors are not significantly different

• The extracellular PTC binding sites between taster and non-taster receptors are not significantly different

• Non-tasting in AVI haplotype is likely due to V-262 and I-296; aa 49 has minimal impact

• Consistent with PVI = non-taster

• AA sites 262 and 296 have the potential to interfere with TM6 and TM7 interaction (V and I are bulkier than A and V, respectively)

• It is hypothesized that movement of TM6 is necessary for activation of the heterotrimeric G protein

PTC taste sensitivity displays a broad and continuous distribution (e.g., it behaves like a quantitative trait).

• PTC taste sensitivity displays a broad and continuous distribution (e.g., it behaves like a quantitative trait).

• On average, PTC taste sensitivity is highest for the PAV/PAV (taster) homozygotes, slightly but significantly lower for the PAV heterozygotes, and lowest by far for the AVI/AVI (non-taster) homozygotes.

• More rare AVI/AAV heterozygotes have a mean PTC score slightly, but significantly, higher than the AVI/AVI homozygotes.

• All non-human primates examined to date are homozygous for the PAV (taster) haplotype. Thus, the AVI nontaster haplotype arose after humans diverged from the most recent common primate ancestor.

• There are non-taster chimps: same gene, but different mutation than humans => molecular convergent evolution!!