Phosphorylated form of MacMARCKS is essential to LFA-1-dependent cell-celladhesion of U937 monocytic cells

MacMARCKS (MRP, F52), a protein kinase C (PKC) substrate, is involved in th e activation of beta 2-integrin. To determine the role of the PKC-mediated phosphorylation of MacMARCKS in this process, human U937 monocytic cells we re transfected with cDNAs encoding wild type or mutant MacMARCKS. We observ ed that the expression of the exogenous wild type MacMARCKS greatly enhance d LFA-I-mediated cell-cell adhesion in U937 cells treated with phorbol 12-m yristate 13-acetate (PMA). This MacMARCKS-stimulated adhesion depended on t he phosphorylation status of MacMARCKS: whereas phosphorylated MacMARCKS en hanced adhesion, unphosphorylated MacMARCKS inhibited it. However, phosphor ylated MacMARCKS alone could not induce LFA-l-mediated cell-cell adhesion u nless phorbol esters were added, suggesting that the phosphorylation of oth er proteins might also be involved. Okadaic acid, a phosphatase inhibitor, induced LFA-l-mediated cell-cell adhesion only in the cells expressing wild type or phosphorylated MacMARCKS and not in the cells expressing unphospho rylated MacMARCKS. Therefore, we conclude that the phosphorylated form of M acMARCKS is essential to LFA-l-mediated cell-cell adhesion. (C) 1999 Wiley- Liss, Inc.