Mechanistic insights into an ancient adenovirus precursor protein VII
shows multiple nuclear import receptor pathways
Abstract
Adenoviral pVII proteins are multifunctional, highly basic, histone-like
proteins that can bind to and transport the viral genome into the host
cell nucleus. Despite the identification of several nuclear localization
signals (NLSs) in the pVII protein of human adenovirus 5 (HAdV5), the
mechanistic details of nuclear transport are largely unknown. Here we
provide a full characterization of the nuclear import of precursor
(Pre-) pVII protein from an ancient siadenovirus, frog siadenovirus 1
(FrAdV1) using a combination of structural, functional and biochemical
approaches. Two strong NLSs (termed NLSa and NLSd) interact with
importin (IMP)b and IMPa respectively, and are the main drivers of
nuclear import. A weaker NLS (termed NLSb) also contributes, together
with an additional signal (NLSc) which we found to be important for
nucleolar targeting. Expression of Pre-pVII wild-type and NLS defective
derivatives in the presence of selective inhibitors of different nuclear
import pathways revealed that, unlike its human counterpart, FrAdV1
Pre-pVII nuclear import is dependent on IMPa/b and IMPb1. Clearly, AdVs
evolved to maximise the nuclear import pathways for the pVII proteins,
whose subcellular localization is the result of a complex process.
Therefore, our results pave the way for an evolutionary comparison of
the interaction of different AdVs with the host cell nuclear transport
machinery.