Alzheimer's disease is characterized by a widespread functional disturbance of the human brain. Fibrillar amyloid proteins are deposited inside neurons as neurofibrillary tangles and extracellularly as amyloid plaque cores and in blood vessels. The major protein subunit (A4) of the amyloid fibril of tangles, plaques and blood vessel deposits is an insoluble, highly aggregating small polypeptide of relative molecular mass 4,500. The same polypeptide is also deposited in the brains of aged individuals with trisomy 21 (Down's syndrome). We have argued previously that the A4 protein is of neuronal origin and is the cleavage product of a larger precursor protein. To identify this precursor, we have now isolated and sequenced an apparently full-length complementary DNA clone coding for the A4 polypeptide. The predicted precursor consists of 695 residues and contains features characteristic of glycosylated cell-surface receptors. This sequence, together with the localization of its gene on chromosome 21, suggests that the cerebral amyloid deposited in Alzheimer's disease and aged Down's syndrome is caused by aberrant catabolism of a cell-surface receptor.

The precursor of the Alzheimer's disease‐specific amyloid A4 protein is an integral, glycosylated membrane protein which spans the bilayer once. The carboxy‐terminal domain of 47 residues was located at the cytoplasmic site of the membrane. The three domains following the transient signal sequence of 17 residues face the opposite side of the membrane. The C‐terminal 100 residues of the precursor comprising the amyloid A4 part and the cytoplasmic domain have a high tendency to aggregate, and proteinase K treatment results in peptides of the size of amyloid A4. This finding suggests that there is a precursor‐product relationship between precursor and amyloid A4 and we conclude that besides proteolytic cleavage other events such as post‐translational modification and membrane injury are primary events that precede the release of the small aggregating amyloid A4 subunit.

The promoter of the gene for the human precursor of Alzheimer's disease A4 amyloid protein (PAD gene) resembles promoters of housekeeping genes. It lacks a typical TATA box and shows a high GC content of 72% in a DNA region that confers promoter activity to a reporter gene in an in vivo assay. Transcription initiates at multiple sites. Sequences homologous to the consensus binding sites of transcription factor AP‐1 and the heat shock control element binding protein were found upstream of the RNA start sites. Six copies of a 9‐bp‐long GC‐rich element are located between positions −200 and −100. A protein–DNA interaction could be mapped to this element. The 3.8 kb of the 5′ region of the PAD gene include two Alu‐type repetitive sequences. These findings suggest that four mechanisms may participate in the regulation of the PAD gene and could be of relevance for the progression of amyloid deposition in Alzheimer's disease.

The nucleotide sequences of the gal E gene coding for UDP-galactose-4-epimerase and the gal T gene coding for galactose-1-P uridyltransferase of Escherichia coli have been determined. UDP-galactose-4-epimerase and galactose-1-P uridyltransferase are predicted to consist of 338 and 347 residues, respectively, NH2-terminal methionines included.

Alzheimer's disease (AD) is characterized by the cerebral deposition of fibrillar aggregates of the amyloid A4 protein. Complementary DNA's coding for the precursor of the amyloid A4 protein have been described. In order to identify the structure of the precursor gene relevant clones from several human genomic libraries were isolated. Sequence analysis of the various clones revealed 16 exons to encode the 695 residue precursor protein (PreA4(695] of Alzheimer's disease amyloid A4 protein. The DNA sequence coding for the amyloid A4 protein is interrupted by an intron. This finding supports the idea that amyloid A4 protein arises by incomplete proteolysis of a larger precursor, and not by aberrant splicing.

Cloning and sequence analysis revealed the putative amyloid A4 precursor (pre‐A4) of Alzheimer's disease to have characteristics of a membrane‐spanning glycoprotein. In addition to brain, pre‐A4 mRNA was found in adult human muscle and other tissues. We demonstrate by in situ hybridization that pre‐A4 mRNA is present in adult human muscle, in cultured human myoblasts and myotubes. Immunofluorescence with antipeptide antibodies shows the putative pre‐A4 protein to be expressed in adult human muscle and associated with some but not all nuclear envelopes. Despite high levels of a single 3.5‐kb pre‐A4 mRNA species in cultured myoblasts and myotubes, the presence of putative pre‐A4 protein could not be detected by immunofluorescence. This suggests that putative pre‐A4 protein is stabilized and therefore functioning in the innervated muscle tissue but not in developing, i.e. non‐innervated cultured muscle cells. The selective localization of the protein on distinct nuclear envelopes could reflect an interaction with motor endplates.

No abstract