The Kelch motif is a region of protein sequence found widely in proteins from bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 and eukaryotes. The motif is named after the Drosophila

Drosophila

Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

 mutant in which it was first identified. This sequence motif is composed of about 50 amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 residues which form a structure of a four stranded beta-sheet "blade". This sequence motif is found in between six and eight copies per protein which associate to form a larger structure called a beta-propeller domain

Protein domain

A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...

.

Proteins containing Kelch motifs

The Kelch motif is widely found in eukaryotic and bacterial species. Notably the human genome contains around 100 proteins containing the Kelch motif. Within individual proteins the motif occurs multiple times. For example, the motif appears 6 times in Drosophila

Drosophila

Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called "fruit flies" or more appropriately pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many species to linger around overripe or rotting fruit...

 egg-chamber regulatory protein. The motif is also found in mouse protein MIPP and in a number of poxviruses. In addition, kelch repeats have been recognised in alpha- and beta-scruin, and in galactose oxidase from the fungus Dactylium dendroides.

Structure

The structure of galactose oxidase reveals that the repeated Kelch sequence motif corresponds to a 4-stranded anti-parallel beta-sheet motif that forms the repeat unit in a super-barrel structural fold commonly known as a beta propeller.

Function

The known functions of kelch-containing proteins are diverse:

• scruin is an actin cross-linking protein;
• galactose oxidase
  Galactose oxidase
  In enzymology, a galactose oxidase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are D-galactose and O2, whereas its two products are D-galacto-hexodialdose and H2O2....
  
   catalyses the oxidation of the hydroxyl group at the C6 position in D-galactose;
• neuraminidase
  Neuraminidase
  Neuraminidase enzymes are glycoside hydrolase enzymes that cleave the glycosidic linkages of neuraminic acids. Neuraminidase enzymes are a large family, found in a range of organisms. The most commonly known neuraminidase is the viral neuraminidase, a drug target for the prevention of the spread...
  
   hydrolyses sialic acid residues from glycoproteins;
• kelch may have a cytoskeletal function, as it is localised to the actin-rich ring canals that connect the 15 nurse cells to the developing oocyte
  Oocyte
  An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
  
  in Drosophila.