Extensin
Encyclopedia
Extensins are a family of flexuous, rodlike, hydroxyproline
-rich glycoprotein
s (HRGPs) of the plant cell wall, discovered by Derek T.A. Lamport at the University of Cambridge
.
They are highly abundant proteins. There are around 20 extensins in Arabidopsis thaliana
. They form crosslinked networks in the young cell wall. Typically they have two major diagnostic repetitive peptide motifs, one hydrophilic and the other hydrophobic, with potential for crosslinking. Extensins are thought to act as self-assembling
amphiphiles essential for cell-wall assembly and growth by cell extension and expansion. The name "extensin" encapsulates the hypothesis that they are involved in cell extension, and some scientists consider it a misnomer.
(Ser) and four hydroxyproline
s (Hyp): Ser-Hyp-Hyp-Hyp-Hyp. Hydroxyproline is unusual not only as a cyclic amino acid that restricts peptide flexibility but as an amino acid with no codon, being encoded as proline
. Polypeptides targeted for secretion
are subsequently hydroxylated by direct addition of molecular oxygen to proline at C-4. Extensin hydroxyproline is uniquely glycosylated with short chains of L-arabinose that further rigidify and increase hydrophilicity. Generally the serine has a single galactose attached.
s separated by a single amino acid, typically valine or another tyrosine, form a short intra-molecular diphenylether crosslink. This can be crosslinked further by the enzyme extensin peroxidase to form an inter-molecular bridge between extensin molecules and thus form networks and sheets.
Hydroxyproline
-4-Hydroxyproline, or L-hydroxyproline , is a common non-proteinogenic amino acid, abbreviated as HYP, e.g., in Protein Data Bank.-Structure and discovery:...
-rich glycoprotein
Glycoprotein
Glycoproteins are proteins that contain oligosaccharide chains covalently attached to polypeptide side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glycosylation. In proteins that have segments extending...
s (HRGPs) of the plant cell wall, discovered by Derek T.A. Lamport at the University of Cambridge
University of Cambridge
The University of Cambridge is a public research university located in Cambridge, United Kingdom. It is the second-oldest university in both the United Kingdom and the English-speaking world , and the seventh-oldest globally...
.
They are highly abundant proteins. There are around 20 extensins in Arabidopsis thaliana
Arabidopsis thaliana
Arabidopsis thaliana is a small flowering plant native to Europe, Asia, and northwestern Africa. A spring annual with a relatively short life cycle, arabidopsis is popular as a model organism in plant biology and genetics...
. They form crosslinked networks in the young cell wall. Typically they have two major diagnostic repetitive peptide motifs, one hydrophilic and the other hydrophobic, with potential for crosslinking. Extensins are thought to act as self-assembling
Self-assembly
Self-assembly is a term used to describe processes in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction...
amphiphiles essential for cell-wall assembly and growth by cell extension and expansion. The name "extensin" encapsulates the hypothesis that they are involved in cell extension, and some scientists consider it a misnomer.
Hydrophilic motif
This pentapeptide consists of serineSerine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...
(Ser) and four hydroxyproline
Hydroxyproline
-4-Hydroxyproline, or L-hydroxyproline , is a common non-proteinogenic amino acid, abbreviated as HYP, e.g., in Protein Data Bank.-Structure and discovery:...
s (Hyp): Ser-Hyp-Hyp-Hyp-Hyp. Hydroxyproline is unusual not only as a cyclic amino acid that restricts peptide flexibility but as an amino acid with no codon, being encoded as proline
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
. Polypeptides targeted for secretion
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
are subsequently hydroxylated by direct addition of molecular oxygen to proline at C-4. Extensin hydroxyproline is uniquely glycosylated with short chains of L-arabinose that further rigidify and increase hydrophilicity. Generally the serine has a single galactose attached.
Hydrophobic tyrosine crosslinking motif
Two tyrosineTyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...
s separated by a single amino acid, typically valine or another tyrosine, form a short intra-molecular diphenylether crosslink. This can be crosslinked further by the enzyme extensin peroxidase to form an inter-molecular bridge between extensin molecules and thus form networks and sheets.