Role of MacMARCKS in Integrin-dependent Macrophage Spreading and Tyrosine Phosphorylation of Paxillin

The cellular function of the MARCKS family of protein kinase C substrates is unknown. In this report, we present evidence that indicates a role for MacMARCKS, a member of the MARCKS family, in the integrin-dependent signal transduction pathways in macrophages. Using a dominant negative mutant of MacMARCKS, we showed that MacMARCKS participates in several integrin-dependent macrophage functions, including the phorbol ester-stimulated macrophage spreading, a process involving multiple integrins. The dominant negative mutant also blocks macrophage spreading on immune complex-coated surfaces, a process again requiring beta2 integrin. More direct evidence of the role of MacMARCKS in the integrin-dependent pathway is the ablation of macrophage binding to complement iC3b-coated sheep erythrocytes by MacMARCKS mutant, suggesting an effect of this mutant on the avidity of complement receptor 3, a member of the beta2 integrin family. To further evaluate the possible mechanism of MacMARCKS function, the integrin-dependent tyrosine phosphorylation of paxillin was examined. Concomitant with the inhibition of macrophage spreading and rosette formation, MacMARCKS mutant also inhibits integrin-dependent tyrosine phosphorylation of paxillin. Furthermore, immunofluorescent microscopy data showed that MacMARCKS and paxillin colocalize in the membrane ruffles at the leading edge of the spreading cells, providing a potential site and opportunity for MacMARCKS to participate in the regulation of integrin-dependent tyrosine phosphorylation of paxillin. Together, these data strongly suggest that MacMARCKS plays a role in integrin-dependent signal transduction pathways in macrophages.