Cysteine-rich proteins (CRP, cysteine-rich peptide or disulphide-rich peptide) are small proteins that contain a large number of cysteines. These cysteines either

cross-link In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...

to form

disulphide bonds In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

, or bind metal ions by

chelation Chelation () is a type of bonding of ions and their molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These l ...

, stabilising the protein's tertiary structure. CRPs include a highly conserved secretion peptide signal at the N-terminus and a cysteine-rich region at the C-terminus.

Structure

Disulphides

In an oxidising environment cysteines cross-link to form

. CRPs that form these typically have an even number of cysteines.

Metal binding

Cysteines can coordinate one or more metal ions by forming a

complex around them.

Functions in plants

CRPs are numerous in plants, with 756 CRP-encoding genes in the ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small plant from the mustard family (Brassicaceae), native to Eurasia and Africa. Commonly found along the shoulders of roads and in disturbed land, it is generally ...

'' genome. Several CRPs bind known receptors, but most CRP signaling mechanisms and protein interactions are uncharacterized. Characterized CRPs function as short-range intercellular signals during processes such as plant defense, bacterial symbiosis, stomatal patterning, fertilization, vegetative tissue development, and seed development. Many CRPs function in plant defense.

Defensins Defensins are small cysteine-rich cationic proteins across cellular life, including vertebrate and invertebrate animals, plants, and fungi. They are host defense peptides, with members displaying either direct antimicrobial activity, immune si ...

, a major class of CRP with an eight-cysteine motif forming four

disulfide bridges In chemistry, a disulfide (or disulphide in British English) is a compound containing a functional group or the anion. The linkage is also called an SS-bond or sometimes a disulfide bridge and usually derived from two thiol groups. In inorg ...

, are involved in pathogen response. Other putative antimicrobial CRPs include

lipid transfer proteins Plant lipid transfer proteins, also known as plant LTPs or PLTPs, are a group of highly- conserved proteins of about 7-9 kDa found in higher plant tissues. As its name implies, lipid transfer proteins facilitate the shuttling of phospholipids and ...

, thionins, knottins, heveins, and snakins. Additionally, some CRPs have allergenic, ɑ-amylase inhibitory, or protease inhibitory functions that deter herbivores. In plant reproduction, CRPs are involved in

pollen tube A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells fr ...

growth and guidance and early embryo patterning, in addition to other functions. Among those involved in pollen tube attraction are the LUREs, a group of ovular pollen-tube attractants in ''Arabidopsis thaliana'' and ''Torenia fournieri'' that preferentially attract conspecific pollen, and STIG1, a CRP expressed in the stigma of ''

Solanum lycopersicum The tomato (, ), ''Solanum lycopersicum'', is a plant whose fruit is an edible berry that is eaten as a vegetable. The tomato is a member of the nightshade family that includes tobacco, potato, and chili peppers. It originated from and was d ...

'' that interacts with the pollen-specific receptor PRK2. In early embryo development, CRPs such as ESF1 are necessary for suspensor development and normal seed morphology.

References

{{Reflist Proteins Sulfides Protein classification Cysteine-rich proteins