Cross-linking of cell surface receptors by multivalent ligands, e.g. by antibodies, evokes their clustering - patching. Subsequently, these clusters can be translocated by the acto-myosin machinery toward one pole of the cell and assembly cap. Patching of FcγRII in U937 cells correlates with tyrosine phosphorylation of several proteins while cap assembly correlates with their dephosphorylation. To study the mechanism of activation of tyrosine kinases during FcγRII activation we disturbed the organization of the putative plasma membrane microdomains by depletion of membrane cholesterol and sphingomyelin. Cholesterol was removed with the use of β-cyclodextrin while sphingomyelin was decomposed by exogenous sphingomyelinase. Cyclodextrin at 5-10 mM removed about 70% of cholesterol from the cells and abolished the assembly of FcγRII caps thereby arresting the receptors at the patching stage. Similarly, 70 mU/ml sphingomyelinase inhibited cap formation by 60%. Cholesterol and sphingomyelin depletion also suppressed the tyrosine phosphorylation of proteins which accompanied cross-linking of FcγRII. The observations indicate that cholesterol and sphingomyelin can control the interactions of tyrosine kinases with clustered FcγRII.
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