Placental alkaline phosphatase is efficiently targeted to rafts in supported lipid bilayers.
Evidence is growing that biological membranes contain lipid microdomains or "rafts" that may be involved in processes such as cellular signaling and protein trafficking. In this study, we have used atomic force microscopy to examine the behavior of rafts in supported lipid bilayers. We show that bilayers composed of equimolar dioleoylphosphatidylcholine and sphingomyelin spontaneously form rafts, which are detectable as raised features. A comparison of the extents of protrusion of the rafts in monolayers and bilayers indicates that the rafts in the two leaflets of the bilayer coincide. The rafts were observed both in the absence and presence of cholesterol (33 mol %). Cholesterol reduced raft protrusion presumably by increasing the thickness of the non-raft bilayer. PLAP (glycosylphosphatidylinositol-anchored protein placental alkaline phosphatase) was purified and shown to exist as a dimer. Following its incorporation into supported lipid bilayers, PLAP was found to be targeted efficiently to rafts, both in the absence and presence of cholesterol. We suggest that atomic force microscopy provides a powerful tool for the study of raft structure and properties.
Saslowsky
David E
DE
Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1PD, UK.
Lawrence
Jared
J
Ren
Xiaoyan
X
Brown
Deborah A
DA
Henderson
Robert M
RM
Edwardson
J Michael
JM
D000469:Alkaline Phosphatase / Q000378:metabolism*; D000818:Animals; D001921:Brain / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism; D008051:Lipid Bilayers; D021962:Membrane Microdomains / Q000378:metabolism* / Q000648:ultrastructure; D018625:Microscopy, Atomic Force; D010713:Phosphatidylcholines / Q000378:metabolism; D010920:Placenta / Q000201:enzymology*; D013109:Sphingomyelins / Q000378:metabolism
D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D013109:Sphingomyelins; D002784:Cholesterol; D000469:Alkaline Phosphatase