Efficient complementation by chimeric Microviridae internal scaffolding proteins is a function of the COOH-terminus of the encoded protein.

Microviridae morphogenesis is dependent on two scaffolding proteins, an internal and external species. Both structural and genetic analyses suggest that the COOH-terminus of the internal protein is critical for coat protein recognition and specificity. To test this hypothesis, chimeric internal scaffolding genes between Microviridae members phiX174, G4, and alpha3 were constructed and the proteins expressed in vivo. All of the chimeric proteins were functional in complementation assays. However, the efficient complementation was observed only when the viral coat protein and COOH-terminus of internal scaffolding were of the same origin. Genes with 5' deletions of the phiX174 internal scaffolding gene were also constructed and expressed in vivo. Proteins lacking the first 10 amino acids, which self-associate across the twofold axes of symmetry in the atomic structure, efficiently complement phiX174 am(B) mutants at temperatures above 24 degrees C. These results suggest that internal scaffolding protein self-associations across the twofold axes of symmetry are required only at lower temperatures.