During infection, pathogenic bacteria often transfer virulence factors to host cells and utilize adherence mechanisms to persist. We detected closely related adhesion and virulence factor-like proteins within the multicellular phototrophic symbiotic consortium `Chlorochromatium aggregatum´, indicating a specific role in mutual interactions. This highly developed microbial symbiosis between two species of different phyla represents the only bacterial model consortium that is experimentally accessible to date. Within `C. aggregatum´, the phototrophic epibiotic Chlorobium encodes three unique RTX-like or hemagglutinin-like proteins, each found to be directly involved in the interaction. Two proteins are of giant size, up to 36,000 amino acids long, thereby representing the largest proteins yet discovered in any organism, surpassing the previous record of titin protein in humans. Growth kinetic-dependent transcriptomics substantiated the expression of both giant proteins and respective secretion systems during the symbiotic state. Notably, immunoblotting, superresolution immunofluorescence microscopy, and immunogold labelling localized each epibiont-produced symbiosis factor predominantly at either the junction regions with its heterotrophic partner or within the cells of the latter. Structural and homology prediction of the RTX-like protein as a biofilm alginate lyase (allowing nutrient exchange between bacteria) in addition to its structural adhesion potential, was also supported by enzymatic assays from heterologous over-expression extracts. Analogous genes were further found in uncultured metagenomes of similar consortia "Pelochromatium roseum". Together, our results suggest that hemagglutinin-like and RTX-like proteins are also involved in bacterial symbioses, extending far beyond known specific pathogenic interactions with eukaryotes, thereby broadening our general understanding of the evolution of bacterial virulence factors.