Lysosomal acid phosphatase is transported to lysosomes via the cell surface

Lysosomal acid phosphatase (LAP) is transported as a transmembrane protein to dense lysosomes. The pathway of LAP to lysosomes includes the passage through the plasma membrane. LAP is transported from the trans- Golgi to the cell surface with a half-time of < 10 min. Cell surface LAP is rapidly internalized. Most of the internalized LAP is transported back to the cell surface. On average, each LAP molecule cycles > 15 times between the cell surface and the endosomes before it is transferred to dense lysosomes. At equilibrium ~ 4 times more LAP precursor is present in endosomes than at the cell surface. Exposing cells to reduced temperature or weak bases such as NH4Cl, chloroquine and primaquine decreases the steady-state concentration of LAP at the cell surface. The recycling pathway is operative at -20°C and does not include passage of the Golgi/trans-Golgi network. LAP is transferred with a half-time of 5-6 h from the plasma membrane/endosome pool to dense lysosomes, from where it does not recycle to the endosome/plasma membrane pool at a measurable rate.