The ST motif is a commonly occurring feature in

proteins 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 ...

and

polypeptides Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A p ...

. It consists of four or five

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

residues with either

serine Serine (symbol Ser or S) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated − form under biological conditions), a carboxyl group (which is in the deprotonated − form un ...

threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), a carboxyl group (which is in the deprotonated −COO� ...

as the first residue (residue ''i''). It is defined by two internal

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

s. One is between the side chain oxygen of residue ''i'' and the main chain NH of residue ''i'' + 2 or ''i'' + 3; the other is between the main chain oxygen of residue ''i'' and the main chain NH of residue ''i'' + 3 or ''i'' + 4. Two websites are available for finding and examining ST motifs in proteins, Motivated Proteins: and PDBeMotif. When one of the hydrogen bonds is between the main chain oxygen of residue ''i'' and the side chain NH of residue ''i'' + 3 the motif incorporates a

beta turn β turns (also β-bends, tight turns, reverse turns, Venkatachalam turns) are the most common form of turns—a type of non-regular secondary structure in proteins that cause a change in direction of the polypeptide chain. They are very common mot ...

. When one of the hydrogen bonds is between the side chain oxygen of residue ''i'' and the main chain NH of residue ''i'' + 2 the motif incorporates an

ST turn The ST turn is a structural feature in proteins and polypeptides. Each consists of three amino acid residues (labeled ''i'', ''i'' + 1 and ''i'' + 2) in which residue ''i'' is a serine (S) or threonine (T) that forms a hydrogen b ...

. As with

s, a significant proportion of ST motifs occur at the

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 ...

of an

alpha helix The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues e ...

with the serine or threonine as the

N cap The term N cap (N-cap, Ncap) describes an amino acid in a particular position within a protein or polypeptide.{{cite journal, last=Leader, first=DP, author2=Milner-White EJ , title=The structure of the ends of helices in globular proteins, journal= ...

residue. They have thus often been described as helix capping features. A related motif is the

asx motif The Asx motif is a commonly occurring feature in proteins and polypeptides. It consists of four or five amino acid residues with either aspartate or asparagine as the first residue (residue i). It is defined by two internal hydrogen bonds. One is b ...

which has

aspartate Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...

asparagine Asparagine (symbol Asn or N) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form under biological conditions), an α-carboxylic acid group (which is in the depro ...

as the first residue. Two well conserved

s at α-helical

N-termini 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 amin ...

occur as ST motifs and form part of the characteristic

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecules wi ...

binding sites of SF1 and SF2 type DNA and RNA

helicases Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separati ...

. It has been suggested that the sequences SPXX or STXX are frequently found at DNA-binding sites and also that they are recognized as substrates by some protein kinases. Structural studies of polypeptides indicate that such tetrapeptides can adopt the hydrogen bonding pattern of the ST motif.{{cite journal, last=Song, first=B, author2=Bomar MG, author3=Kibler P, author4=Kodukula K, author5=Galande AK, title=The Serine-Proline Turn:A novel hydrogen-bonded template for designing peptidomimetics, journal=Organic Letters, year=2012, volume=14, issue=3, pages=732–735, doi=10.1021/ol203272k, pmid=22257322

References

Protein structural motifs