Structure of the scaffold in bacteriophage lambda preheads removal of the scaffold leads to a change of the prehead shell

Abstract

Small angle X-ray scattering was performed on unprocessed and processed preheads, intermediates in the morphogenesis of bacteriophage λ heads. Unprocessed preheads possess an internal structure (scaffold), necessary for efficient assembly of closed shells. Processed preheads, formed after removal of the scaffold, are able to pack and cut the viral DNA in vitro. Our data show that the scaffold fills out the inside of the shell in an almost (but not completely) homogeneous fashion; structures of the scaffold with the bulk of the mass in a small core inside the shell can be excluded. Unprocessed preheads are larger than processed ones. A change in shell architecture takes place upon transition from unprocessed to processed prehead; the shell becomes roughened up. Shrinking of the shell as well as roughening up can be triggered by accidental partial degradation of the scaffold. The lattice constant of type A polyheads is in agreement with the lattice constant derived from our icosahedral models of the shell, indicating a close relationship between processed preheads and type A polyheads. This observation, together with the type of subunit clustering found, leads us to propose a simple model for the interaction of prehead shell and protein p D, which stabilizes phage DNA after packaging.