Barstar is a small protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 synthesized by the bacterium Bacillus amyloliquefaciens

Bacillus amyloliquefaciens

Bacillus amyloliquefaciens is a species of bacteria that is the source of the BamH1 restriction enzyme. It also synthesizes a natural antibiotic protein barnase, a widely studied ribonuclease that forms a famously tight complex with its intracellular inhibitor barstar.-Discovery and name:B...

. Its function is to inhibit the ribonuclease

Ribonuclease

Ribonuclease is a type of nuclease that catalyzes the degradation of RNA into smaller components. Ribonucleases can be divided into endoribonucleases and exoribonucleases, and comprise several sub-classes within the EC 2.7 and 3.1 classes of enzymes.-Function:All organisms studied contain...

 activity of its binding partner barnase

Barnase

Barnase is a bacterial protein that consists of 110 amino acids and has ribonuclease activity. It is synthesized and secreted by the bacterium Bacillus amyloliquefaciens, but is lethal to the cell when expressed without its inhibitor barstar...

, with which it forms an extraordinarily tightly bound complex within the cell until barnase is secreted. Expression of barstar is necessary to counter the lethal effect of expressed active barnase. The structure of the barnase-barstar complex is known.