Domain architecture of the bacteriophage phi29 connector protein.

Viral connectors are essential components of the DNA packaging machinery. They interact with nucleic acids and other viral components to translocate DNA inside the viral head. We have attempted to locate the different structural and functional domains of the phage Phi29 connector using a combination of approaches to generate different antigenic probes. Complexes of native connectors with either monoclonal or monospecific antibodies were studied by immunoelectron microscopy and image averaging methods. The data were merged in a model of the connector domain structure at 2-3 nm resolution. This epitope mapping provides a general outline of the folding architecture of the connector polypeptide, following a complicated threading that places the amino and carboxyl-terminals in close alignment in the narrower domain at 2-3 nm from the top of the connector. The appendages are built up by a long and highly immunogenic sequence (amino acid residues 153 to 206). The RNA binding domain forms part of the top of the narrow conical area of the connector, a flexible region that undergoes structural changes during viral morphogenesis. The DNA binding domain is located not far away, 2-3 nm below, in the outer side of the narrow conical part. The precise location of the functional domains of the connector, as well as their relative positions provide the first experimental framework for understanding the connector function.