Smith, B. O. | bioRxiv Science

bioRxiv2021

Tuning the sensitivity of genetically encoded fluorescent potassium indicators through structure-guided and genome mining strategies

Genetically encoded potassium indicators lack optimal binding affinity for monitoring intracellular dynamics in mammalian cells. Through structure-guided design and genome mining of potassium binding proteins, we developed green fluorescent potassium indicators with a broad range of binding affinities. KRaION1, based on the insertion of a potassium binding protein (Ec-Kbp) into the fluorescent protein mNeonGreen, exhibits an isotonically measured Kd of 69{+/-}10 (mM; mean {+/-} standard deviation used throughout). We identified Ec-Kbps binding site using NMR spectroscopy to detect protein-thallium scalar couplings and refined the structure of Ec-Kbp in its potassium-bound state. Guided by this structure, we modified KRaION1, yielding KRaION2, which exhibits an isotonically measured Kd of 96{+/-}9 (mM). We identified four Ec-Kbp homologs as potassium binding proteins, which yielded indicators with isotonically measured binding affinities in the 39-112 (mM) range. KRaIONs expressed and functioned in HeLa cells, but exhibited lower Kd values, which were mirrored by lower Kd values measured in vitro when holding sodium constant. Thus, potassium indicator Kd may need to be evaluated in the context of a given experimental goal.

bioengineering↗

bioRxiv2021

CK2 phosphorylation of human papillomavirus 16 E2 on serine 23 promotes interaction with TopBP1 and is critical for E2 plasmid retention function

During the human papillomavirus 16 (HPV16) life cycle, the E2 protein interacts with host factors to regulate viral transcription, replication and genome segregation/retention. Our understanding of host partner proteins and their roles in E2 functions remains incomplete. Here, we demonstrate that CK2 phosphorylation of E2 on serine 23 promotes interaction with TopBP1 in vitro and in vivo, and that E2 is phosphorylated on this residue during the HPV16 life cycle. We investigated the consequences of mutating serine 23 on E2 functions. E2-S23A activates and represses transcription identically to E2-WT (wild-type), and E2-S23A is as efficient as E2-WT in transient replication assays. However, E2-S23A has compromised interaction with mitotic chromatin when compared with E2-WT. In E2-WT cells, both E2 and TopBP1 levels increase during mitosis when compared with vector control cells. In E2-S23A cells, neither E2 nor TopBP1 levels increase during mitosis. We next tested whether this difference in E2-S23A levels during mitosis disrupts E2 plasmid retention function. We developed a novel plasmid retention assay and demonstrate that E2-S23A is deficient in plasmid retention when compared with E2-WT. siRNA targeted knockdown of TopBP1 abrogates E2-WT plasmid retention function. Introduction of the S23A mutation into the HPV16 genome resulted in delayed immortalization of human foreskin keratinocytes (HFK) and higher episomal viral genome copy number in resulting established HFK. Overall, our results demonstrate that CK2 phosphorylation of E2 on serine 23 promotes interaction with TopBP1, which is critical for E2 plasmid retention function and in HPV16 immortalization of keratinocytes. ImportanceHuman papillomaviruses are causative agents in around 5% of all cancers, with no specific anti-viral therapeutics available for treating infections or resultant cancers. In this report, we demonstrate that phosphorylation of HPV16 E2 by CK2 promotes formation of a complex formation with the cellular protein TopBP1 in vitro and in vivo. This complex results in stabilization of E2 during mitosis and mediates plasmid retention by E2. This function promotes the partitioning of viral genomes into the nuclei of daughter cells following mitosis. We demonstrate that CK2 phosphorylates E2 on serine 23 in vivo, and that CK2 inhibitors disrupt the E2-TopBP1 complex. Mutation of E2 serine 23 to alanine disrupts the HPV16 life cycle, demonstrating a critical function for this residue. Together, our results suggest that CK2 inhibitors may disrupt the E2-TopBP1 dependent HPV16 life cycle and potentially kill HPV16 positive cancers, which lays a molecular foundation to develop novel therapeutic approaches for combating HPV16 disease.

molecular biology↗