Attaches the circulating virion to the bacterial lipopolysaccharides which serve as receptor for the virus. Determines the phage host-range. Probably triggers with protein H the injection of the phage DNA into the host cytoplasm upon conformational changes induced by the interaction with host lipopolysaccharides. Pentamerizes and interacts with H protein, F and B pentamers to form 12S pre-assembly complex. Joining of twelve 12S complex form the procapsid. Belongs to the microvirus G protein family.