Cysteine-rich proteins (also cysteine-rich peptide, CRP, disulphide-rich peptide) are small proteins that contain a large number of cysteines. These cysteines either cross-link to form

disulphide bonds In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

, or bind metal ions by chelation, stabilising the protein's

tertiary structure Protein tertiary structure is the three dimensional shape of a protein. The tertiary structure will have a single polypeptide chain "backbone" with one or more protein secondary structures, the protein domains. Amino acid side chains may int ...

. 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 chelation 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 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 a ...

'' 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, a major class of CRP with an eight-cysteine motif forming four disulfide bridges, 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 7-9kDa found in higher plant tissues. As its name implies, lipid transfer proteins facilitate the shuttling of phospholipids and o ...

thionins Thionins are a family of small proteins found solely in higher plants. Typically, a thionin consists of 45–48 amino acid residues. 6–8 of these are cysteine forming 3–4 disulfide bonds. They include phoratoxins and viscotoxins. Alpha- and ...

, 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 fro ...

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'' 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