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, re ...

and

polypeptides Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty ami ...

. It consists of four or five

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

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 − ...

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 when dissolved in water), 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 (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

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 amin ...

of an

alpha helix An alpha helix (or α-helix) is a sequence of amino acids in a protein that are twisted into a coil (a helix). The alpha helix is the most common structural arrangement in the Protein secondary structure, secondary structure of proteins. It is al ...

with the serine or threonine as the N cap 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 ...

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. The L-isomer of aspartic acid is one of the 22 proteinogenic amino acids, i.e., the building blocks of protein ...

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 occur as ST motifs and form part of the characteristic

nucleotide Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

binding sites of SF1 and SF2 type DNA and RNA helicases. 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 A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a fun ...

. 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