phiX174 utilizes two scaffolding proteins during morphogenesis, an internal protein (B) and an external protein (D). The B protein induces a conformational 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 functions of the amino terminus, several 5' deleted genes were constructed and the proteins expressed in vivo. The DeltaNH(2) 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 biochemical, 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.