Uncoupling the functions of a multifunctional protein: The isolation of a DNA pilot protein mutant that affects particle morphogenesis

Defective øX174 H protein-mediated DNA piloting indirectly influences the entire viral lifecycle. Faulty piloting can mask the H protein's other functions or inefficient penetration may be used to explain defects in post-piloting phenomena. For example, optimal synthesis of other viral proteins requires de novo H protein biosynthesis. As low protein concentrations affect morphogenesis, protein H's assembly functions remain obscure. An H protein mutant was isolated that allowed morphogenetic effects to be characterized independent of its other functions. The mutant protein aggregates assembly intermediates. Although excess internal scaffolding protein restores capsid assembly, the resulting mutant H protein-containing particles are less infectious. In addition, nonviable phenotypes of am(H) mutants in Su⁺ hosts, which insert non-wild-type amino acids, do not always correlate with a lack of missense protein function. Phenotypes are highly influenced by host and phage physiology. This phenomenon was unique to am(H) mutants, not observed with amber mutants in other genes.