The pentatricopeptide repeat (PPR) is a 35-

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

sequence motif In biology, a sequence motif is a nucleotide or amino-acid sequence pattern that is widespread and usually assumed to be related to biological function of the macromolecule. For example, an ''N''-glycosylation site motif can be defined as ''A ...

. Pentatricopeptide-repeat-containing proteins are a family of

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

commonly found in the

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

kingdom. They are distinguished by the presence of tandem degenerate PPR motifs and by the relative lack of

introns An intron is any Nucleic acid sequence, nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e., a region inside a gene."The notion of ...

in the genes coding for them. Approximately 450 such proteins have been identified in the

Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae. They are small flowering plants related to cabbage and mustard. This genus is of great interest since it contains thale cress (''Arabidopsis thaliana''), one of the model organ ...

genome, and another 477 in the

rice Rice is a cereal grain and in its Domestication, domesticated form is the staple food of over half of the world's population, particularly in Asia and Africa. Rice is the seed of the grass species ''Oryza sativa'' (Asian rice)—or, much l ...

genome. Despite the large size of the protein family, genetic data suggest that there is little or no redundancy of function between the PPR proteins in ''Arabidopsis''. The purpose of PPR proteins is currently under dispute. It has been shown that a good deal of those in ''Arabidopsis'' interact (often essentially) with

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

and other

organelles In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' th ...

and that they are possibly involved in

RNA editing RNA editing (also RNA modification) is a molecular process through which some cells can make discrete changes to specific nucleotide sequences within an RNA molecule after it has been generated by RNA polymerase. It occurs in all living organisms ...

. However many trans proteins are required for this editing to occur and research continues to look at which proteins are needed. The structure of the PPR has been resolved. It folds into a

helix-turn-helix Helix-turn-helix is a DNA-binding domain (DBD). The helix-turn-helix (HTH) is a major structural motif capable of binding DNA. Each monomer incorporates two alpha helix, α helices, joined by a short strand of amino acids, that bind to the majo ...

structure similar to those found in the

tetratricopeptide The tetratricopeptide repeat (TPR) is a structural motif. It consists of a degenerate 34 amino acid tandem repeat identified in a wide variety of proteins. It is found in tandem arrays of 3–16 motifs, which form scaffolds to mediate protein� ...

repeat. Several repeats of the protein forms a ring around a single-strand RNA molecule in a sequence-sensitive way reminiscent of

TAL effector TAL (transcription activator-like) effectors (often referred to as TALEs, but not to be confused with the Homeobox, three amino acid loop extension homeobox class of proteins) are proteins secreted by some β-proteobacteria, β- and γ-proteobacter ...

Examples

Human genes encoding proteins containing this repeat include: * DENND4A, DENND4B, DENND4C * LRPPRC * PTCD1, PTCD2, PTCD3 * MRPS27

References

Amino acid motifs {{molecular-biology-stub