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.

The parasite Plasmodium falciparum causes the most severe form of malaria and to invade and replicate in red blood cells (RBCs), it exports hundreds of proteins across the encasing parasitophorous vacuole membrane (PVM) into this host cell. The exported proteins help modify the RBC to support rapid parasite growth and avoidance of the human immune system. Most exported proteins possess a conserved Plasmodium Export Element (PEXEL) motif with the consensus RxLxE/D/Q amino acid sequence, which acts as a proteolytic cleavage recognition site within the parasite’s endoplasmic reticulum (ER). Cleavage occurs after the P 1 L residue and is thought to help release the protein from the ER so it can be putatively escorted by the HSP101 chaperone to the parasitophorous vacuole space surrounding the intraerythrocytic parasite. HSP101 and its cargo are then thought to assemble with the rest of a Plasmodium Translocon for Exported proteins (PTEX) complex, that then recognises the xE/D/Q capped N terminus of the exported protein and translocates it across the vacuole membrane into the RBC compartment. Here, we present evidence that supports a dual role for the PEXEL’s conserved P 2’ position E/Q/D residue, firstly, for plasmepsin V cleavage in the ER, and secondly, for efficient PTEX mediated export across the PVM into the RBC. We also present evidence that the downstream ‘spacer’ region separating the PEXEL motif from the folded functional region of the exported protein controls cargo interaction with PTEX as well. The spacer must be of a sufficient length and permissive amino acid composition to engage the HSP101 unfoldase component of PTEX to be efficiently translocated into the RBC compartment.