The Functions of the N Terminus of the fX174 Internal Scaffolding Protein, a Protein Encoded in an Overlapping Reading Frame in a Two Scaffolding Protein System

fX174 utilizes two scaffolding proteins during morphogenesis, an internal protein (B) and an external protein (D). The B protein induces a confor-mational change in coat protein pentamers, enabling them to interact with both spike and external scaffolding proteins. While functions of the carboxyl terminus of protein B have been defined, the functions of the amino terminus remain obscure. To investigate the morphogenetic func-tions of the amino terminus, several 50 deleted genes were constructed and the proteins expressed in vivo. The DNH2 B proteins were assayed for the ability to complement an ochre B mutant and defects in the morphogenetic pathway were characterized. The results of the bio-chemical, genetic and second-site genetic analyses indicate that the amino terminus induces conformational changes in the viral coat protein and facilitates minor spike protein incorporation. Defects in conformational switching can be suppressed by substitutions in the external scaffolding protein, suggesting some redundancy of function between the two proteins