The timing and magnitude of Ca2+ signaling by CD32 depends on its redistribution on the cell surface.
Ca2+ signaling was correlated with microaggregation and capping of CD32 molecules on the myeloid cell line, U937. The cytosolic free Ca2+ signal was related to the extent of CD32 cross-linking and arose asymmetrically within individual cells. Both the magnitude and the delay before Ca2+ signaling via CD32 on U937 cells was dependent on the extent of CD32 cross-linking. The delay time was extended in cells in which lateral diffusion in the membrane was reduced by covalently cross-linking of surface proteins. Under these conditions, capping but not surface microaggregation of CD32 molecules was prevented. The delay time before Ca2+ signaling but not the magnitude was also affected. At a higher density of covalent cross-linking of surface proteins, the magnitude of the Ca2+ signal by CD32 was also reduced and could be completely inhibited. This evidence therefore shows that the formation of a CD32 "cap" was not required for Ca2+ signaling by this route. However, the signaling delay time was a consequence of lateral diffusion of CD32 molecules in the membrane to form signaling-competent microaggregates, and the redistribution of CD32 molecules on the cell surface was required for Ca2+ signal generation.
Roberts
G M
GM
Molecular Signalling Group, University Department of Surgery, University of Wales College of Medicine, Heath Park, Cardiff, United Kingdom.
Davies
E V
EV
Pettit
E J
EJ
Hallett
M B
MB
D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D003432:Cross-Linking Reagents; D003600:Cytosol / Q000378:metabolism; D006801:Humans; D017452:Receptors, IgG / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*; D013997:Time Factors; D014407:Tumor Cells, Cultured
D003432:Cross-Linking Reagents; D017452:Receptors, IgG; D002118:Calcium