Involvement of a triton-insoluble floating fraction in Dictyostelium cell-cell adhesion

We have isolated and characterized a Triton-insoluble floating fraction (TIFF) from Dictyostelium. Ten major proteins were consistently detected in TIFF, and six species were identified by mass spectrometry as actin, porin, comitin, regulatory myosin light chain, a novel member of the CD36 family, and the phospholipid-anchored cell adhesion molecule gp80. TIFF was enriched with many acylated proteins. Also, the sterol/phospholipid ratio of TIFF was 10-fold higher than that of the bulk plasma membrane. Immunoelectron microscopy showed that TIFF has vesicular morphology and confirmed the association of gp80 and comitin with TIFF membranes. Several TIFF properties were similar to those of Dictyostelium contact regions, which were isolated as a cytoskeleton-associated membrane fraction. Mass spectrometry demonstrated that TIFF and contact regions shared the same major proteins. During development, gp80 colocalized with F-actin, porin, and comitin at cell-cell contacts. These proteins were also recruited to gp80 caps induced by antibody cross-linking. Filipin staining revealed high sterol levels in both gp80-enriched cell-cell contacts and gp80 caps. Moreover, sterol sequestration by filipin and digitonin inhibited gp80-mediated cell-cell adhesion. This study reveals that Dictyostelium TIFF has structural properties previously attributed to vertebrate TIFF and establishes a role for Dictyostelium TIFF in cell-cell adhesion during development.