Sup35p is the ''
Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been o ...
'' (a
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitut ...
)
eukaryotic
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
translation
Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
release factor
A release factor is a protein that allows for the termination of translation by recognizing the termination codon or stop codon in an mRNA sequence. They are named so because they release new peptides from the ribosome.
Background
During t ...
. More specifically, it is the yeast eukaryotic
release factor
A release factor is a protein that allows for the termination of translation by recognizing the termination codon or stop codon in an mRNA sequence. They are named so because they release new peptides from the ribosome.
Background
During t ...
3 (eRF3), which forms the translation termination complex with eRF1 (
Sup45p Sup45p is the ''Saccharomyces cerevisiae'' (a yeast) eukaryotic translation termination factor. More specifically, it is the yeast eukaryotic release factor 1 (eRF1). Its job is to recognize stop codons in RNA and bind to them. It binds to the Sup3 ...
in yeast). This complex recognizes and catalyzes the release of the nascent polypeptide chain when the
ribosome
Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...
encounters a
stop codon
In molecular biology (specifically protein biosynthesis), a stop codon (or termination codon) is a codon (nucleotide triplet within messenger RNA) that signals the termination of the translation process of the current protein. Most codons in me ...
. While eRF1 recognizes stop codons, eRF3 facilitates the release of the polypeptide chain through GTP hydrolysis.
Partial loss of function results in nonsense suppression, in which stop codons are ignored and
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s are abnormally synthesized with
carboxyl
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
terminal extensions. Complete loss of function is fatal.
History
Sup35p was shown to propagate in a
prion
Prions are misfolded proteins that have the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It ...
form in 1994 by
Reed Wickner
Reed B. Wickner (born c. 1942) is an American yeast geneticist. In 1994 he proposed that the 'PSI''+and RE3phenotypes in ''Saccharomyces cerevisiae'', a form of budding yeast, were caused by prion forms of native proteins - specifically, the Su ...
. For this reason it is an intensely studied protein. When yeast cells harbor Sup35p in the prion state the resulting phenotype is known as
SI+ In
SI+cells Sup35p exists in an
amyloid
Amyloids are aggregates of proteins characterised by a Fibril, fibrillar morphology of 7–13 Nanometer, nm in diameter, a beta sheet (β-sheet) Secondary structure of proteins, secondary structure (known as cross-β) and ability to be Staining, ...
state that can be propagated and passed to daughter cells. This results in less soluble and functional protein and thus in an increased rate of nonsense suppression (translational read-through of stop codons).
The overexpression of the gene has been shown to induce the
si+conformation.
Evolutionary capacitance
Several journal articles have suggested that the ability to interconvert between
SI+and
si-prion-free) states provides an evolutionary advantage, but this remains an area of much debate.
Susan Lindquist
Susan Lee Lindquist, ForMemRS (June 5, 1949 – October 27, 2016) was an American professor of biology at MIT specializing in molecular biology, particularly the protein folding problem within a family of molecules known as heat-shock proteins ...
has shown that
isogenic populations of yeast can express different phenotypes based on whether they had the prion form of Sup35p or the non-prion form. She did an experiment where seven strains of yeast with different genetic backgrounds were grown under many different stressful conditions, with matched
SI+and
si-strains.
In some cases, the
SI+version grew faster, in others
si-grew faster. She proposed that
SI+may act as an
evolutionary capacitor to facilitate adaptation by releasing cryptic genetic variation in natural populations at times of stress. This variation would lie beyond stop codons, which show a high rate of in-frame loss in yeast. Mathematical models suggest that
SI+may have evolved for this function.
Physical Characteristics
Sup 35 contains a carboxyl-terminal region (
C-terminus
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
), which is responsible for the translation-termination activity. The amino-terminal(
N-terminus
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
) region of the protein is responsible for alternately folding depending on the conformation. The middle (m) domain has an unknown function. In an effort to determine the function of these N and M regions, in Susan Lindquists' experiment two of the strains were engineered to produce a version of Sup35p which does not include the N and M regions.
The Sup35p protein is 685 amino acids long. The C-terminal contains 5 complete and one incomplete repeat of the
Oligopeptide repeat sequence PQGGYQQ-YN. In modified versions of the gene, it has been shown that the more repeats of this sequence present, the more the protein is to assume the
si+confirmation. In fact, the addition of two extra repeats (R2) result in the
si-to
si+conversion in being 5000 times faster.
PMN2, a mutant, dominant version of the gene Sup35p, has a glycine to aspartic acid substitution in the second repeat. The resulting
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
is a lack of ability to maintain the
si+conformation.
The N-terminus has a high glutamine/asparagine amount at 43%, while the average yeast protein only contains 9%. The N terminus is 114 amino acids long and is termed the prion forming domain (PrD). Over expression of the Sup35p gene can lead to
si+
Both the N and M terminals and the C terminus form binding sites to Sup45p, giving a total of two. Also, in binding to Sup45p the
si+protein can cause it to aggregate and form a prion.
Adenine Pathway
The phenotypic differences between
si-and
si+is made clear when the ability of the cell to make
adenine
Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its derivati ...
is tampered with. The buildup of P-ribosylamino imidazole (AIR) (a precursor in the
adenine pathway in yeast) induces a red pigment in a yeast colony visible to the naked eye. In isogenic strains where the non-sense mutation is in the middle of either the gene ADE 2 or ADE 1 (enzymes involved in the pathway), the
si-strain has either build ups of P-ribosylamino imidazole (AIR), or P-ribosylamino imidazolecarboxylate (CAIR), respectively. Because CAIR converts back into AIR if the enzyme that catalyzes it to the next precursor is absent, either mutation will cause a red color in the
si-strain. The
si+strain appears white even when subjected to the same non-sense mutations. Thus, it is inferred that the eRF3 of the
si+is non-functional.
This phenomenon is because the eRF3 in
si-is able to disconnect the ribosome effectively, so the enzyme cannot be properly synthesized. However, in the
si+strain, the enzyme is able to be synthesized enough so that the pathway still successfully produces adenine.
See also
*
Translation (genetics)
In molecular biology and genetics, translation is the process in which ribosomes in the cytoplasm or endoplasmic reticulum synthesize proteins after the process of transcription of DNA to RNA in the cell's nucleus. The entire process is ...
*
Fungal Prions
A fungal prion is a prion that infects fungal hosts. Fungal prions are naturally occurring proteins that can switch between multiple, structurally distinct conformations, at least one of which is self-propagating and transmissible to other prions ...
*
Prion
Prions are misfolded proteins that have the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It ...
*
Release factor
A release factor is a protein that allows for the termination of translation by recognizing the termination codon or stop codon in an mRNA sequence. They are named so because they release new peptides from the ribosome.
Background
During t ...
*
Sup45p Sup45p is the ''Saccharomyces cerevisiae'' (a yeast) eukaryotic translation termination factor. More specifically, it is the yeast eukaryotic release factor 1 (eRF1). Its job is to recognize stop codons in RNA and bind to them. It binds to the Sup3 ...
References
{{Reflist
Prions
Saccharomyces cerevisiae genes