What is Amyloid Precursor Protein(APP)?

Amyloid Precursor Protein(APP) is an integral membrane protein expressed in many tissues and concentrated in the synapses of neurons. Its primary function is not known, though it has been implicated as a regulator of synapse formation, neural plasticity and iron export. APP is best known and most commonly studied as the precursor molecule whose proteolysis generates beta amyloid (Aβ), a 39- to 42-amino acid peptide whose amyloid fibrillar form is the primary component of amyloid plaques found in the brains of Alzheimer's disease patients.
Amyloid Precursor Protein(APP) undergoes extensive post-translational modification including glycosylation, phosphorylation, and tyrosine sulfation, as well as many types of proteolytic processing to generate peptide fragments. It is commonly cleaved by proteases in the secretase family; alpha secretase and beta secretase both remove nearly the entire extracellular domain to release membrane-anchored carboxy-terminal fragments that may be associated with apoptosis. Cleavage by gamma secretase within the membrane-spanning domain generates the amyloid-beta fragment; gamma secretase is a large multi-subunit complex whose components have not yet been fully characterized, but include presenilin, whose gene has been identified as a major genetic risk factor for Alzheimer's.
Structure
A number of distinct, largely independently-folding structural domains have been identified in the APP sequence. The extracellular region, much larger than the intracellular region, is divided into the E1 and E2 domains, linked by an acidic domain (AcD); E1 contains two subdomains including a growth factor-like domain (GFLD) and a copper-binding domain (CuBD) interacting tightly together. A serine protease inhibitor domain, absent from the isoform differentially expressed in the brain, is found between acidic region and E2 domain. The complete crystal structure of APP has not yet been solved; however, individual domains have been successfully crystallized, the growth factor-like domain, the copper-binding domain, the complete E1 domain and the E2 domain.
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