Gelsolin functional overexpression impairs HIV-1 Env-gp120-mediated cell-to-cell fusion, regardless of viral Env tropism. Effect of gelsolin-EGFP (A) or free gelsolin (B; co-expressed with free EGFP using an EGFP-pSUPER-gelsolin plasmid) overexpression in HeLa P5 cells on HIV-1 Env-mediated membrane fusion measured by β-Galactosidase production, and referred to the EGFP or EGFP-pSUPER control condition (100% of membrane fusion). Treated HeLa P5 cells were fused with HeLa cells expressing X4-tropic (243) or R5-tropic (ADA) Env, in the presence or absence of an anti-CD4 mAb. The data represent the mean ± s.e.m. of four independent experiments carried out in triplicate: ** p < 0.01, Student’s t-test. Western blot quantification of gelsolin-EGFP (A), free EGFP (A and B) and gelsolin (B; detection of endogenous and overexpressedfree gelsolin) expression together with total α-tubulin levels prior to cell-to-cell fusion, showing representative data for each experimental condition. Cell-surface expression of viral receptors was analyzed under each experimental condition (A and B). Data represent the mean ± s.e.m. of four independent experiments carried out in triplicate.