SPHINGOMYELINASE TREATMENT INDUCES ATP-INDEPENDENT ENDOCYTOSIS

ATP hydrolysis has been regarded as a general requirement for internal ization processes in mammalian cells, We found, however, that treatmen t of ATP-depleted macrophages and fibroblasts with exogenous sphingomy elinase (SMase) rapidly induces formation of numerous vesicles that pi nch off from the plasma membrane; the process is complete within 10 mi n after adding SMase. By electron microscopy, the SMase-induced vesicl es are similar to 400 nm in diameter and lack discernible coats. 15-30 % of plasma membrane is internalized by SMase treatment, and there is no detectable enrichment of either clathrin or caveolin in these vesic les. When ATP is restored to the cells, the SMase-induced vesicles are able to deliver fluid-phase markers to late endosomes/lysosomes and r eturn recycling receptors, such as transferrin receptors, back to the plasma membrane, We speculate that hydrolysis of sphingomyelin on the plasma membrane causes inward curvature and subsequent fusion to form sealed vesicles. Many cell types express a SMase that can be secreted or delivered to endosomes and lysosomes, The hydrolysis of sphingomyel in by these enzymes is activated by several signaling pathways, and th is may lead to formation of vesicles by the process described here.