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J. Cell Biol.,
Volume 140, Number 1, January 12, 1998 39-47
* Department of Pathology, § Department of Medicine, and ATP hydrolysis has been regarded as a general requirement for internalization processes in mammalian cells. We found, however, that treatment of
ATP-depleted macrophages and fibroblasts with exogenous sphingomyelinase (SMase) rapidly induces formation of numerous vesicles that pinch off from the
plasma membrane; the process is complete within 10 min after adding SMase. By electron microscopy, the
SMase-induced vesicles are ~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 vesicles. When ATP is restored to the cells, the
SMase-induced vesicles are able to deliver fluid-phase markers to late endosomes/lysosomes and return 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 sphingomyelin by these enzymes is activated by several signaling pathways, and this may lead
to formation of vesicles by the process described here.
Department of Anatomy and Cell Biology, Columbia University
College of Physicians and Surgeons; and Department of Biochemistry, Cornell University Medical College, New York 10021
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