The activity of joining two lipid bilayers to form a single membrane. [ http://www.ncbi.nlm.nih.gov/pubmed/10332732 http://www.ncbi.nlm.nih.gov/pubmed/32100701 http://www.ncbi.nlm.nih.gov/pubmed/32641474 http://www.ncbi.nlm.nih.gov/pubmed/11493675 http://www.ncbi.nlm.nih.gov/pubmed/12600315 ]