Domain architecture of the bacteriophage Phi 29 connector protein

Viral connectors are essential components of the DNA packaging machinery. T hey interact with nucleic acids and other viral components to translocate D NA inside the viral head. We have attempted to locate the different structu ral and functional domains of the phage Phi 29 connector using a combinatio n 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 mer ged 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 th e 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 lon g 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 con nector, a flexible region that undergoes structural changes during viral mo rphogenesis. 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 f unctional domains of the connector, as well as their relative positions pro vide the first experimental framework for understanding the connector funct ion. (C) 1999 Academic Press.