Component of the tail fiber that acts as receptor binding protein. Binds to the primary receptor, thereby determining the host range. Two alternate tail fiber S and S' proteins are encoded extending the host range of the virus. Tail fiber. Expressed in the late phase of the viral replicative cycle. Expression of late genes is activated by the viral late transcription activator C. Expressed alternatively with tail fiber protein S' (associated with tail fiber assembly protein U'). The switch from S-U to S'-U' occurs through inversion of a DNA segment called G by the phage invertase protein Gin. Viral particles lack tail fibers in inversion-defective gin mutants (blocked in G+ orientation; no switch to S'-U'). The orientation of the G segment is defined as G+ and G-. G+ orientation provides S-U fibers whereas G- provides S'-U' fibers. S-U and S'-U' dont have the same host range (e.g. respectively E.coli and C.freundii). Belongs to the caudovirales tail fiber protein family.