The adenovirus type 5 capsid protein IIIa is phosphorylated during an early stage of infection of HeLa cells.

[32P]Orthophosphate-preincubated and thus 32P-labeled ATP-containing HeLa cells were infected with adenovirus type 5 particles. Following a 30 min infection period, viral particles were recovered and purified from infected cells by means of sucrose gradient and CsCl equilibrium centrifugation. Analysis of such recovered adenovirus particles by gel electrophoresis revealed that the capsid protein IIIa was phosphorylated. Further, a close resemblance was found in the tryptic peptide map of this in vivo phosphorylated IIIa and that of in vitro phosphorylated IIIa obtained from dialyzed virions. These results and other observations suggest that the previously known in vitro phosphorylation of IIIa with experimentally produced penton-free adenovirus particles likely mimics a phenomenon which takes place at an early stage of infection.