Binds to the host phosphopolysaccharides at the onset of infection. Upon activation by calcium, the receptor binding proteins change their conformation, presenting their binding sites to the host, and a channel opens at the bottom of the baseplate for DNA ejection. Homotrimer; found as an assembly of 6 homotrimers (PubMed:20351260, PubMed:24027307). Interacts with host phosphopolysaccharides; this allows the virus to bind the external cell wall layer (PubMed:16547026, PubMed:20351260, PubMed:24027307). Interacts with the distal tail protein (PubMed:24027307). Part of the baseplate. The C-terminus is involved in host receptor binding. Host-recognition domains point upwards, towards the capsid. Belongs to the skunalikevirus receptor binding protein family.