InterPro domain: IPR044815

This family represents the CRINKLE4-related proteins CCR3 and CCR4.

This family includes plant serine/threonine receptor-like kinases related to CRINKLY4 (CR4), a protein involved in developmental processes in plant and endosperm that was first isolated in maize [ 1 , 2 ]. Mutations in this protein affects the cell wall thickness and structure, cuticle formation, and vesicle trafficking, and tumor like outgrowths, with similar effects seen in rice [ 3 , 3 , 3 ]. Arabidopsis thaliana contains an orthologue of CR4, ACR4, and four CRINKLY4-related proteins (CRR or CCR) AtCRR1, AtCRR2, AtCRR3 and AtCRR4 (also known as CRINKLY 4-related kinase 1, AtCRK1) [ 4 ]. Phylogenetic analysis showed that the CR4 family of receptor kinases can be divided in three clades, one including CR4, CCR1 and CCR2, a second including CCR3 and CCR4 family members, and a third and more distant clade including members from algae and Selaginella moellendorffii sequences with transmembrane and/or kinase domains [ 4 ]. Kinase assays showed that ACR4 is an active serine/threonine kinase, while CCR1 and CCR2 are nearly inactive in autophosphorylation assays [ 4 ].

CR4 family are characterised by the presence of seven 'crinkly' repeats in the extracellular part which is required both for signalling and normal protein internalisation, including a conserved C(X~10)CWG sequence motif. The Cys residues in the extracellular 'crinkly' repeat domain are likely to form stabilizing disulfide bridges [ 4 ]. Another feature of the CR4 family is that the extracellular domain shows homology to the three Cys-rich repeats of the TUMOR NECROSIS FACTOR RECEPTOR (TNFR) extracellular domain [ 4 ].

1. Molecular analysis of the CRINKLY4 gene family in Arabidopsis thaliana. Planta 220, 645-57
2. The phenotype of the CRINKLY4 deletion mutant of Physcomitrella patens suggests a broad role in developmental regulation in early land plants. Planta 244, 275-84
3. A phylogenetic approach to study the origin and evolution of the CRINKLY4 family. Front Plant Sci 6, 880