Functional molecular complexes of human N-formyl chemoattractant receptors and actin

When human neutrophils become desensitized to formyl peptide chemoattractants, the receptors (FPR) for these peptides are converted to a high affinity, GTP-insensitive form that is associated with the Triton X-100-insoluble membrane skeleton from surface membrane domains. These domains are actin and fodrin-rich, but G protein-depleted suggesting that FPR shuttling between G protein-enriched and depleted domains may control signal transduction. To determine the molecular basis for FPR interaction with the membrane skeleton, neutrophil subcellular fractions were screened for molecules that could bind photoaffinity-radioiodinated FPR solubilized in Triton X-100. These receptors showed a propensity to bind to a 41- to 43-kDa protein band on nitrocellulose overlays of SDS-PAGE-separated cytosol and plasma membrane fractions of neutrophils. This binding, as well as FPR binding to purified neutrophil actin, was inhibited 50% by 0.6 microM free neutrophil cytosolic actin. Addition of greater than 1 microM G-actin to crude or lectin-purified Triton X-100 extracts of FPR from neutrophil membranes increased the sedimentation rate of a significant fraction of FPR two to three fold as measured by velocity sedimentation in Triton X-100-containing linear sucrose density gradients. Addition of anti-actin antibodies to FPR extracts caused a concentration-dependent immunoprecipitation of at least 65% of the FPR. More than 40% of the immunoprecipitated FPR was specifically retained on protein A affinity matrices. Membrane actin was stabilized to alkaline washing when membranes were photoaffinity labeled. Conversely, when purified neutrophil cytosolic actin was added to membranes or their digitonin extracts, after prior depletion of actin by an alkaline membrane wash, photoaffinity labeling of FPR was increased two- to fourfold with an EC50 of approximately 0.1 microM actin. We conclude that FPR from human neutrophils may interact with actin in membranes to form Triton X-100-stable physical complexes. These complexes can accept additional G-actin monomers to form higher order molecular complexes. Formation of FPR-actin complexes in the neutrophil may play a role in the regulation of chemoattractant-induced activation or actin polymerization.