Structural Dynamics and Activity of B19V VP1u during the pHs of Cell Entry and Endosomal Trafficking

Parvovirus B19 (B19V) is a human pathogen that is the causative agent of fifth disease in children. It is also known to cause hydrops in fetuses, anemia in AIDS patients, and transient aplastic crisis in patients with sickle cell disease. The unique N-terminus of Viral Protein 1 (VP1u) of parvoviruses, including B19V, exhibits phospholipase A₂ (PLA₂) activity, which is required for endosomal escape. Presented is the structural dynamics of B19V VP1u under conditions that mimic the pHs of cell entry and endosomal trafficking to the nucleus. Using circular dichroism spectroscopy, the receptor-binding domain of B19V VP1u is shown to exhibit an α-helical fold, whereas the PLA₂ domain exhibits a probable molten globule state, both of which are pH invariant. Differential scanning calorimetry performed at endosomal pHs shows that the melting temperature (T_m) of VP1u PLA₂ domain is tuned to body temperature (37 °C) at pH 7.4. In addition, PLA₂ assays performed at temperatures ranging from 25-45 °C show both a temperature and pH-dependent change in activity. We hypothesize that VP1u PLA₂ domain differences in T_m at differing pHs have enabled the virus to "switch on/off" the phospholipase activity during capsid trafficking. Furthermore, we propose the environment of the early endosome as the optimal condition for endosomal escape leading to B19V infection.