Polyglutamine tract

A polyglutamine tract or polyQ tract is a portion of a protein consisting of a sequence of several glutamine units. A tract typically consists of about 10 to a few hundred such units.

Several genes, both in humans and in other species, contain a number of repetitions of the nucleotide triplet CAG. When the gene is translated into a protein, each of these triplets gives rise to a glutamine unit, resulting in a polyglutamine tract. Different alleles of such a gene often have different numbers of CAG triplets.

Several inheritable neurodegenerative disorders, the polyglutamine diseases, occur if a mutation causes a polyglutamine tract in a specific gene to become too long. Important examples of polyglutamine diseases are spinocerebellar ataxia and Huntington's disease. It is believed that cells cannot properly dispose of proteins with overly long polyglutamine tracts, which over time leads to damage in nerve cells. The longer the polyglutamine tract, the earlier in life these diseases tends to appear.