Sup35p
Encyclopedia
Sup35p is the Saccharomyces cerevisiae
(a yeast
) eukaryotic translation
release factor
. More specifically, it is the yeast eukaryotic release factor
3 (eRF3), which forms the translation termination complex with eRF1 (Sup45p
in yeast). This complex recognizes and catalyzes the release of the nascent polypeptide chain when the ribosome
encounters a stop codon
. 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
s are abnormally synthesized with carboxyl terminal extensions. Complete loss of function is fatal.
form in 1994 by Reed Wickner
. For this reason it is an intensely studied protein. When yeast cells harbor Sup35p in the prion state the resulting phenotype is known as [PSI+]. In [PSI+] cells Sup35p exists in an amyloid
state that can be propagated and passed to daughter cells. This results in less soluble and functional protein and thus an in increased rate of nonsense suppression (translational read-through of stop codons).
The overexpression of the gene has been shown to induce the [Psi+] conformation.
Susan Lindquist
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 [PSI+] and [psi-] strains. In some cases, the [PSI+] version grew faster, in others [psi-] grew faster. She proposed that [PSI+] 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 [PSI+] may have evolved for this function.
The Sup35p protein is 201 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 [Psi+] confirmation. In fact, the addition of two extra repeats (R2) result in the [Psi-] to [Psi+] 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
is a lack of ability to maintain the [Psi+] 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 [Psi+].
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 [psi+] protein can cause it to aggregate and form a prion.
is tampered with. The build up 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 [psi-] 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 [psi-] strain. The [psi+] strain appears white even when subjected to the same non-sense mutations. Thus, it is inferred that the eRF3 of the [psi+] is non-functional.
This phenomenon is because the eRF3 in [psi-] is able to disconnect the ribosome effectively so that the enzyme cannot be properly synthesized. However, in the [psi+] strain, the enzyme is able to be synthesized enough so that the pathway still successfully produces adenine.
Saccharomyces cerevisiae
Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skin of grapes...
(a yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...
) eukaryotic translation
Translation (genetics)
In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
release factor
Release factor
A release factor is a protein that allows for the termination of translation by recognizing the termination codon or stop codon in a mRNA sequence....
. More specifically, it is the yeast eukaryotic release factor
Release factor
A release factor is a protein that allows for the termination of translation by recognizing the termination codon or stop codon in a mRNA sequence....
3 (eRF3), which forms the translation termination complex with eRF1 (Sup45p
Sup45p
Sup45p is the Saccharomyces cerevisiae eukaryotic translation termination factor. More specifically, it is the yeast eukaryotic release factor 1 . Its job is to recognize stop codons in RNA and bind to them...
in yeast). This complex recognizes and catalyzes the release of the nascent polypeptide chain when the ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
encounters a stop codon
Stop codon
In the genetic code, a stop codon is a nucleotide triplet within messenger RNA that signals a termination of translation. Proteins are based on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA correspond to the addition of an amino acid to a growing polypeptide...
. 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
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s are abnormally synthesized with carboxyl terminal extensions. Complete loss of function is fatal.
History
Sup35p was shown to propagate in a prionPrion
A prion is an infectious agent composed of protein in a misfolded form. This is in contrast to all other known infectious agents which must contain nucleic acids . The word prion, coined in 1982 by Stanley B. Prusiner, is a portmanteau derived from the words protein and infection...
form in 1994 by Reed Wickner
Reed Wickner
Reed B. Wickner is an American yeast geneticist. In 1994 he proposed that the [PSI+] and [URE3] phenotypes in Saccharomyces cerevisiae, a form of budding yeast, were caused by prion forms of native proteins. Specifically, the sup35p protein....
. For this reason it is an intensely studied protein. When yeast cells harbor Sup35p in the prion state the resulting phenotype is known as [PSI+]. In [PSI+] cells Sup35p exists in an amyloid
Amyloid
Amyloids are insoluble fibrous protein aggregates sharing specific structural traits. Abnormal accumulation of amyloid in organs may lead to amyloidosis, and may play a role in various neurodegenerative diseases.-Definition:...
state that can be propagated and passed to daughter cells. This results in less soluble and functional protein and thus an in increased rate of nonsense suppression (translational read-through of stop codons).
The overexpression of the gene has been shown to induce the [Psi+] conformation.
Evolutionary capacitance
Several recent journal articles have suggested that the ability to interconvert between [PSI+] and [psi-](prion-free) states provides an evolutionary advantage, but this remains an area of much debate.Susan Lindquist
Susan Lindquist
Susan Lindquist is a professor of biology at MIT specializing in molecular biology, particularly the protein folding problem within a family of molecules known as heat-shock proteins, and prions...
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 [PSI+] and [psi-] strains. In some cases, the [PSI+] version grew faster, in others [psi-] grew faster. She proposed that [PSI+] 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 [PSI+] may have evolved for this function.
Physical Characteristics
Sup 35 contains a carboxyl-terminal region (C-terminus), which is responsible for the translation-termination activity. The amino-terminal(N-terminus) 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 strains were engineered to produce a version of Sup35p which does not include the N and M regions.The Sup35p protein is 201 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 [Psi+] confirmation. In fact, the addition of two extra repeats (R2) result in the [Psi-] to [Psi+] 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
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
is a lack of ability to maintain the [Psi+] 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 [Psi+].
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 [psi+] protein can cause it to aggregate and form a prion.
Adenine Pathway
The phenotypic differences between [psi-] and [psi+] is made clear when the ability of the cell to make adenineAdenine
Adenine is a nucleobase with a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate and the cofactors nicotinamide adenine dinucleotide and flavin adenine dinucleotide , and protein synthesis, as a chemical component of DNA...
is tampered with. The build up of P-ribosylamino imidazole (AIR) (a precursor in the adenine pathway
Adenine pathway
The Adenine Pathway in yeast is a seven step pathway converting P-ribosyl-PP to adenine. This product is then either integrated into DNA, RNA, or a ribose diphosphate molecule ADP, the precursor of ATP, an essential intermediate storage molecule for most cellular processes.-Mechanism:The ADE series...
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 [psi-] 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 [psi-] strain. The [psi+] strain appears white even when subjected to the same non-sense mutations. Thus, it is inferred that the eRF3 of the [psi+] is non-functional.
This phenomenon is because the eRF3 in [psi-] is able to disconnect the ribosome effectively so that the enzyme cannot be properly synthesized. However, in the [psi+] strain, the enzyme is able to be synthesized enough so that the pathway still successfully produces adenine.
See also
- TranslationTranslation (genetics)In molecular biology and genetics, translation is the third stage of protein biosynthesis . In translation, messenger RNA produced by transcription is decoded by the ribosome to produce a specific amino acid chain, or polypeptide, that will later fold into an active protein...
- Fungal PrionsFungal prionsFungal prions provide an excellent model for the understanding of disease-forming mammalian prions. Fungal prions are naturally occurring proteins that can undergo a structural conversion that becomes self-propagating and infectious. They represent an epigenetic phenomenon in which information is...
- PrionPrionA prion is an infectious agent composed of protein in a misfolded form. This is in contrast to all other known infectious agents which must contain nucleic acids . The word prion, coined in 1982 by Stanley B. Prusiner, is a portmanteau derived from the words protein and infection...
- Release factorRelease factorA release factor is a protein that allows for the termination of translation by recognizing the termination codon or stop codon in a mRNA sequence....
- Sup45pSup45pSup45p is the Saccharomyces cerevisiae eukaryotic translation termination factor. More specifically, it is the yeast eukaryotic release factor 1 . Its job is to recognize stop codons in RNA and bind to them...