Head-Tail Connector Proteins gp6 and gp15, and similar proteins
Members of this family include the proteins gp6 and gp15 from bacteriophage HK97 and SPP1, respectively. They are critical in the assembly of the connector, a specialized structure that serves as an interface for head and tail attachment, as well as a point at which DNA exits the head during infection by the bacteriophage. They form dodecameric ring structures that comprise the middle ring of the connector, located between the portal protein (attached to the head) and the gp7/gp16 ring (attached to the tail). They are components of the mature phage and the absence or mutation of HK97 gp6 or SPP1 gp15, respectively, result in defective head-tail joining and the absence of mature phage particles. The genome maps of HK97 and SPP1 show that genes encoding gp6 and gp15 are in the same relative position on the genome, located adjacent to the major capsid protein (MCP) gene and in between head and tail genes. Also included in this family is the uncharacterized Bacillus subtilis Yqbg protein, whose gene is part of the unusual genetic element called skin. The Yqbg gene is surrounded with genes similar to genes in the Bacillus subtilis prophage-like element PBSX, which encode for proteins comprising contractile-tailed phage-like particles that are produced upon mitomycin C treatment. Yqbg likely acts as a head-tail connector protein, similar to gp6 and gp15, of the PBSX-like prophage encoded in the skin element.