Journal of Molecular Evolution

, Volume 34, Issue 6, pp 536-543

Evolutionary origin of a Kunitz-type trypsin inhibitor domain inserted in the amyloid β precursor protein of Alzheimer's disease

  • Kazuho IkeoAffiliated withDNA Research Center, National Institute of GeneticsThe Graduate University for Advanced Studies
  • , Kei TakahashiAffiliated withDepartment of Physiology, Shimane Medical University
  • , Takashi GojoboriAffiliated withDNA Research Center, National Institute of Genetics

Rent the article at a discount

Rent now

* Final gross prices may vary according to local VAT.

Get Access


The Kunitz-type protease inhibitor is one of the serine protease inhibitors. It is found in blood, saliva, and all tissues in mammals. Recently, a Kunitz-type sequence was found in the protein sequence of the amyloid β precursor protein (βAPP). It is known that βAPP accumulates in the neuritic plaques and cerebrovascular deposits of patients with Alzheimer's disease. Collagen type VI in chicken also has an insertion of a Kunitz-type sequence. To elucidate the evolutionary origin of these insertion sequences, we constructed a phylogenetic tree by use of all the available sequences of Kunitz-type inhibitors. The tree shows that the ancestral gene of the Kunitz-type inhibitor appeared about 500 million years ago. Thereafter, this gene duplicated itself many times, and some of the duplicates were inserted into other protein-coding genes. During this process, the Kunitz-type sequence in the present βAPP gene diverged from its ancestral gene about 270 million years ago and was inserted into the gene soon after duplication. Although the function of the insertion sequences is unknown, our molecular evolutionary analysis shows that these insertion sequences in βAPP have an evolutionarily close relationship with the inter-α-trypsin inhibitor or trypstatin, which inhibits the activity of tryptase, a novel membrane-bound serine protease in human T4+ lymphocytes.

Key words

Serine protease inhibitor Kunitz type Evolutionary origin Alzheimer's disease Insertion sequence