Structure-Function Analysis of the ϕX174 DNA-Piloting Protein Using Length-Altering Mutations

Area of Science:

• Structural biology
• Virology
• Molecular genetics

Background:

• The bacteriophage ϕX174 H protein is essential for DNA translocation into the host cell.
• During procapsid morphogenesis, the H protein is monomeric, but 10 units oligomerize to form the DNA translocating H-tube.

Purpose of the Study:

• To investigate the timing and location of H-tube formation.
• To analyze the structure-function relationship of the H protein using genetic methods.

Main Methods:

• In-frame insertions and deletions were used to create length-altered H proteins.
• Mutant proteins were assessed for their ability to participate in particle assembly, genome translocation, and stimulate viral protein synthesis.

Main Results:

• Length-altered H proteins demonstrated impaired DNA translocation, with lengthened proteins being less infectious.
• Shortened H proteins were not fully functional, with some failing to incorporate into virions, suggesting an 85-amino-acid incorporation domain.
• Revertants indicated sequence duplication or suppression of incorporation defects.

Conclusions:

• The H-tube is likely not fully assembled within the virion before DNA penetration.
• The H protein's repetitive structure facilitates genetic analysis of supramolecular assemblies.
• This study provides a paradigm for understanding viral DNA translocation mechanisms and nanotube formation.