A ROLE FOR MARCKS, THE ALPHA-ISOZYME OF PROTEIN-KINASE-C AND MYOSIN-IIN ZYMOSAN PHAGOCYTOSIS BY MACROPHAGES

Myristoylated, alanine-rich C-kinase substrate (MARCKS) is a lipopolys accharide-induced protein kinase C (PKC) substrate that has been propo sed to regulate actin-membrane interactions, as well as actin structur e at the membrane. We studied the distribution of MARCKS, the alpha is ozyme of PKC (PKC alpha), and myosin I in lipopolysaccharide-treated p eritoneal macrophages ingesting zymosan particles. MARCKS, PKC alpha, and myosin I colocalized with F-actin and talin in the cortical cytopl asm adjacent to forming phagocytic cups. After particle ingestion was completed, myosin I, F-actin, and talin were no longer enriched in the vicinity of the phagosome. By contrast, MARCKS and PKC alpha remained associated with the phagosome membrane until after acquisition of the lysosomal marker Lamp-1. Vinculin was not detected on phagosomes at a ny time point examined. Phagocytosis of zymosan was accompanied by rap id and sustained phosphorylation of MARCKS. Inhibitors of PKC reduced zymosan binding to the macrophage surface and blocked the focal accumu lation of F-actin, talin, phosphotyrosine-containing proteins, MARCKS, and PKC alpha beneath attached particles. We propose that PKC-depende nt phosphorylation is an early signal required for zymosan phagocytosi s and that MARCKS and PKC alpha have a role in phagosome maturation. T he colocalization of F-actin and MARCKS at the cytoplasmic face of the nascent phagosome reinforces the hypothesis that MARCKS regulates act in structure at the membrane. Our data also suggest that myosin I func tions as a mechanical motor during particle uptake.