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 less than 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 greater than 15 times between the cell surface and the endosomes before it is transferred to dense lysosomes. At equilibrium approximately 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 greater than or equal to 20 degrees 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.