Human lysosomal acid phosphatase is transported as a transmembrane protein to lysosomes in transfected baby hamster kidney cells

BHK cells transfected with human lysosomal acid phosphatase (LAP) cDNA (CT29) expressed 70-fold higher enzyme activities of acid phosphatase than nontransfected BHK cells. The CT29-LAP was synthesized in BHK cells as a heterogeneously glycosylated precursor that was tightly membrane associated. Transfer to the trans-Golgi was associated with a small increase in size (~7 kd) and partial processing of the oligosaccharides to complex type structures. CT29-LAP was transferred into lysosomes as shown by subceliular fractionation, immunofluorescence and immunoelectron microscopy. Lack of mannose-6phosphate residues suggested that transport does not involve mannose-6phosphate receptors. Part of the membrane-associated CT29-LAP was processed to a soluble form. The mechanism that converts CT29-LAP into a soluble form was sensitive to NH4Cl, and reduced the size of the polypeptide by 7 kd. In vitro translation of CT29-derived cRNA in the presence of microsomal membranes yielded a CT29-LAP precursor that is protected from proteinase K except for a small peptide of -2 kd. In combination with the sequence data available for LAP, these observations suggest that CT29-LAP is synthesized and transported to lysosomes as a transmembrane protein. In the lysosomes, CT29-LAP is released from the membrane by proteolytic cleavage, which removes a C-terminal peptide including the transmembrane domain and the cytosolic tail of 18 amino acids.