Cysteine-rich protein

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 to form disulphide bonds, or bind metal ions by chelation, stabilising the protein's tertiary structure.^[1][2][3] CRPs include a highly conserved secretion peptide signal at the N-terminus and a cysteine-rich region at the C-terminus.^[4]

Structure

Example disulphide-linked CRP: a thionein. Cysteines in yellow. (PDB: 1BHP​)

Example metal binding CRP: a metallothionein bound to zinc ions. Cysteines in yellow, zinc in purple. (PDB: 1JJD​)

Disulphides

In an oxidising environment cysteines cross-link to form disulphide bonds. CRPs that form these typically have an even number of cysteines.^[5]

Metal binding

Cysteines can coordinate one or more metal ions by forming a chelation complex around them.^[6]

Functions in plants

CRPs are numerous in plants, with 756 CRP-encoding genes in the Arabidopsis thaliana genome.^[7] Several CRPs bind known receptors,^[8] 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.^[4]

Many CRPs function in plant defense. Defensins, a major class of CRP with an eight-cysteine motif forming four disulfide bridges,^[9] are involved in pathogen response.^[4] Other putative antimicrobial CRPs include lipid transfer proteins, thionins, knottins, heveins, and snakins. Additionally, some CRPs have allergenic, ɑ-amylase inhibitory, or protease inhibitory functions that deter herbivores.^[9]

In plant reproduction, CRPs are involved in pollen tube growth and guidance^[10] and early embryo patterning,^[11] 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^[12] that preferentially attract conspecific pollen,^[10] and STIG1, a CRP expressed in the stigma of Solanum lycopersicum that interacts with the pollen-specific receptor PRK2.^[8] In early embryo development, CRPs such as ESF1 are necessary for suspensor development and normal seed morphology.^[11]