Extensins are a family of flexuous, rodlike,

hydroxyproline (2''S'',4''R'')-4-Hydroxyproline, or L-hydroxyproline ( C5 H9 O3 N), is an amino acid, abbreviated as Hyp or O, ''e.g.'', in Protein Data Bank. Structure and discovery In 1902, Hermann Emil Fischer isolated hydroxyproline from hydrolyzed gelatin ...

-rich

glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as g ...

s (HRGPs) of the

plant cell wall A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mec ...

. They are highly abundant proteins. There are around 20 extensions in ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter ...

''. They form crosslinked networks in the young cell wall. Typically they have two major diagnostic repetitive peptide motifs, one

hydrophilic A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press. In contrast, hydrophobes are ...

and the other

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, ...

, with potential for crosslinking. Extensins are thought to act as self-assembling

amphiphile An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compou ...

s 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.Lamport,D.T.A. (1963) Oxygen fixation into hydroxyproline of plant cell wall protein. J.Biol.Chem., 238, 1438-1440.

Hydrophilic motif

This pentapeptide consists of

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 − for ...

(Ser) and four

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 440px Secretion is the movement of material from one point to another, such as a secreted chemical substance from a cell or gland. In contrast, excretion is the removal of certain substances or waste products from a cell or organism. The classica ...

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 Arabinose is an aldopentose – a monosaccharide containing five carbon atoms, and including an aldehyde (CHO) functional group. For biosynthetic reasons, most saccharides are almost always more abundant in nature as the "D"-form, or structurally ...

that further rigidify and increase hydrophilicity. Generally the serine has a single galactose attached.

Hydrophobic tyrosine crosslinking motif

Two

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...

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 peroxidaseSchnabelrauch,L.S., Kieliszewski,M.J., Upham,B.L., Alizedeh,H., and Lamport,D.T.A. (1996) Isolation of pI 4.6 extensin peroxidase from tomato cell suspension cultures and identification of Val-Tyr-Lys as putative intermolecular cross-link site. Plant J., 9, 477-489. to form an inter-molecular bridge between extensin molecules and thus form networks and sheets.

Hydrophilic motif

Hydrophobic tyrosine crosslinking motif

References

Further reading