The CST complex is a cellular

multiprotein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multienzyme complexes, in which multiple catalytic domains are found in a single polypeptide chain. Protein c ...

involved in

telomere A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...

maintenance. In

budding yeast ''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 ...

(''

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 ...

''), it is composed of the

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 Cdc13, Stn1, and Ten1; in

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s, it consists of the proteins CTC1, STN1, and TEN1. It is related to the

Replication protein A Replication protein A (RPA) is the major protein that binds to single-stranded DNA (ssDNA) in eukaryotic cells. In vitro, RPA shows a much higher affinity for ssDNA than RNA or double-stranded DNA. RPA is required in replication, recombinatio ...

complex.

Structure

For budding yeast as well as for mammals, CST is a protein heterotrimer, consisting of three distinct proteins. Yeast Stn1 and Ten1 are

orthologous Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a spec ...

proteins to mammalian STN1 and TEN1. But yeast Cdc13 and mammalian CTC1 are very different in amino acid sequence, length, and to some extent in function.

Function

For both budding yeast and mammals, the CST complex contributes to

maintenance, but this function is more crucial for budding yeast, where the CST complex performs the functions that

shelterin Shelterin (also called telosome) is a protein complex known to protect telomeres in many eukaryotes from DNA repair mechanisms, as well as to regulate telomerase activity. In mammals and other vertebrates, telomeric DNA consists of repeating doubl ...

performs in vertebrates. At least four factors contribute to telomere maintenance:

telomerase Telomerase, also called terminal transferase, is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3' end of telomeres. A telomere is a region of repetitive sequences at each end of the chromosomes of most euka ...

, shelterin,

TERRA Terra may often refer to: * Terra (mythology), primeval Roman goddess * An alternate name for planet Earth, as well as the Latin name for the planet Terra may also refer to: Geography Astronomy * Terra (satellite), a multi-national NASA scienti ...

and the CST Complex. CST protection of telomeres for mammals occurs under conditions of

replication stress DNA replication stress refers to the state of a cell whose genome is exposed to various stresses. The events that contribute to replication stress occur during DNA replication, and can result in a stalled replication fork. There are many events t ...

. But when not replicating DNA, mammals primarily require shelterin for telomere protection. T-loops and

G-quadruplex In molecular biology, G-quadruplex secondary structures (G4) are formed in nucleic acids by sequences that are rich in guanine. They are helical in shape and contain guanine tetrads that can form from one, two or four strands. The unimolecular ...

es are described as the two tertiary DNA structures that protect telomere ends and regulate telomere length. In fungus, the CST complex has been shown to unfold higher order G-tailed structures, such as occur with telomere exposure during

DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part for biological inheritanc ...

References

{{Reflist Protein complexes Telomere-binding proteins

Structure

Function

See also

References