Phosphorylation induces structural changes in the Autographa californica nucleopolyhedrovirus P10 protein

Baculoviruses encode a variety of auxiliary proteins that are not essential for viral replication but provide them with a selective advantage in nature. P10 is a 10 kDa auxiliary protein produced in the very-late phase of gene transcription by Autographa californica multiple nucleopolyhedrovirus (AcMNPV). The P10 protein forms cytoskeletal-like structures in the host cell that associate with microtubules varying from filamentous forms in the cytoplasm to aggregated peri-nuclear tubules that form a cage-like structure around the nucleus. These P10 structures may have a role in the release of occlusion bodies (OBs) and thus mediate horizontal transmission of the virus between insect hosts. Here it is demonstrated, using mass spectrometric analysis, that the C-terminus of P10 is phosphorylated during virus infection of cells in culture. Analysis of the P10 mutants encoded by recombinant baculoviruses in which putative phosphorylation residues were mutated to alanine showed that serine 93 is a site of phosphorylation. Confocal microscopy examination of the serine 93 mutant structures revealed an aberrant formation of the peri-nuclear tubules. Thus, phosphorylation of serine 93 may induce aggregation of filaments to form tubules. Together, these data suggest that the phosphorylation of serine 93 affects P10 structural conformation.IMPORTANCE The baculovirus P10 protein has been researched intensively since it was first observed in 1969, but its role during the viral infection remains unclear. It is conserved in the alphabaculoviruses and expressed at high levels during virus infection. Producing large amounts of a protein is wasteful for the virus unless it is advantageous for survival of its progeny and therefore P10 presents an enigma. As P10 polymerises to form organised cytoskeletal structures that co-localise with the host cell microtubules, the structural relationship of the protein with the host cell may present a key to help understand the function and importance of this protein. This study addresses the importance of the structural changes in P10 during infection and how they may be governed by phosphorylation. The P10 structures affected by phosphorylation are closely associated with the viral progeny and thus, potentially, be responsible for its dissemination and survival