HUMAN AMYLOID PRECURSOR-LIKE PROTEIN-1 - CDNA CLONING, ECTOPIC EXPRESSION IN COS-7 CELLS AND IDENTIFICATION OF SOLUBLE FORMS IN THE CEREBROSPINAL-FLUID

Amyloid precursor-like protein 1 (APLP1) represents an integral membra ne type 1 protein of unknown function which was originally cloned from a mouse cDNA library on the basis of sequence similarity with the Alz heimer's amyloid precursor protein (APP). Here we report on the molecu lar cloning and expression of the human APLP1 (hAPLP1). hAPLP1 consist s of 650 amino acids, displays 89% identity on the amino acid level to its mouse homologue and has a calculated molecular mass of 72 kDa, hA PLP1 synthesized in a cell-free system displays an apparent molecular mass of approximate to 80 kDa in SDS-containing gels and becomes N-gly cosylated when the in vitro translation is performed in the presence o f microsomes. The hAPLP1 cDNA was also expressed ectopically in COS-7 cells and the protein expression was analyzed by immunoprecipitation a nd western blotting. We have demonstrated that hAPLP1 represents a nov el glycoprotein which carries both N- and O-linked glycans. Moreover, hAPLP1 undergoes limited proteolysis which results in the secretion of the carboxy-terminal truncated molecule into the cells conditioned me dium. Examination of cells transfected with hAPLP1 cDNA by confocal la ser microscopy reveals an intense perinuclear and Golgi staining, a pa ttern resembling the subcellular distribution of APP. Using a novel hA PLP1-specific antiserum, we identified soluble hAPLP1 in the human cer ebrospinal fluid, which suggests that secretion of hAPLP1 from brain c ells also takes place in vivo.