The versatility of Ascorbate Peroxidase-aided mapping uncovers insights of thenuclear lamina interactions and function

The nuclear lamina (NL) is a proteinaceous network found beneath the inner nuclear membrane. The NL is linked to a number of dynamic cellular activities including chromatin organization, transcription and RNA/protein trafficking through nuclear pores. Our understanding of the NL has been hindered in part by the general insolubility and low extractability of proteins from this region. This has spurred the development of proximity ligation methods that label proteins and/or DNA near the NL for systematic identification (Bar et al., 2018; Chen et al., 2018b; Guelen et al., 2008; Roux et al., 2012). To simplify labeling and improve temporal resolution, we fused APEX2 (Hung et al., 2014; Lam et al., 2015) to the nuclear lamina protein lamin-B1 to map proteins, RNA and DNA associated with the NL. We show that APEX2 labeling of the NL is robust and requires as little as 20 seconds. In addition to identifying the NL proteome, this method revealed NL-proximal RNA species that were largely spliced. These NL-proximal RNAs show a bias toward long 3 UTRs, suggesting an RNA-regulatory role of the NL. This is further supported by the finding of a bias toward longer 3 UTRs in genes deregulated in lamin-null cells. Interestingly, these RNAs share a sequence motif in their 3 UTRs. Finally, we demonstrate that the APEX2 method can reliably map lamina-associated domains (LADs) at different stages of the cell cycle, revealing a variability of short LADs regions enriched for histone lysine 27 trimethylation (H3K27me3). Thus the APEX2 method report here is a useful addition to the molecular toolbox for the study of the NL and permits the identification of proteome, transcriptome, and genome elements associated with this nuclear substructure.