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 φX174, G4, and α3 were constructed and the proteins expressed in vivo. All of the chimetic proteins were functional in complementation assays. However, the efficient complementation was observed only when the vital coat protein and COOH- terminus of internal scaffolding were of the same origin. Genes with 5' deletions of the φX174 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 φX174 am(B) mutants at temperatures above 24°C. These results suggest that internal scaffolding protein self-associations across the twofold axes of symmetry are required only at lower temperatures. (C) 2000 Academic Press.
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