Abstract
Phagocytosis of pathogens and inert particles such as zymosan by macrophages, and related secretory functions require the combination of several intracellular signals and the reorganization of cytoskeleton. We recently reported that zymosan stimulated the tyrosine phosphorylations of several endogenous substrates in human monocytes. In this work, the relationship between zymosan-stimulated tyrosine phosphoproteins and detergent-insoluble material considered as cytoskeleton was investigated. Triton X-100-insoluble fraction contained two proteins of 53 and 56 kDa that were tyrosine phosphorylated after only 5 min of stimulation with zymosan and remained labeled for 30 min. Because 53- and 56-kDa phosphoproteins migrated, as did some components of the src tyrosine kinase family, namely p53-56lyn, we wondered if 53- and 56-kDa phosphoproteins were related to lyn kinase. First, the amount of immunoreactive p53-56lyn increased in Triton X-100-insoluble fraction as did zymosan-stimulated tyrosine phosphoproteins. This property of p53-56lyn was unique, as no other member of the src family was found in this fraction. Second, when the immunoblots were reprobed with anti-phosphotyrosine mAb, the m.w. of p53-56lyn and tyrosine-phosphorylated proteins were identical in apparent size. Third, p53-56lyn was probably activated after cell stimulation with zymosan, because the phosphorylation levels of a synthetic copolymer of glutamine-tyrosine were increased in Triton X-100-insoluble fraction. In addition, we studied the distribution of lyn kinase and tyrosine phosphoproteins in phagocytozing monocytes. By using immunofluorescence, we showed that lyn kinase was located preferentially in the periphagosomal region in a specific manner, as an src tyrosine kinase such as p59hck, which was not associated with cytoskeleton, was not concentrated around the vacuoles. Moreover, periphagosomal phosphoproteins were also detected and found to be colocalized with polymerized actin. Because zymosan interacts with human monocytes via beta 2 integrins, which are known to be cytoskeleton-associated, we suggest that p53-56lyn provides the molecular link between zymosan receptors and cytoskeleton, and directs the cytoskeletal reorganization in the periphagosomal area.
