Hierarchical TPR:ZC3HC1 interactions drive stepwise assembly of serially arrayed nuclear basket appendages extending into vertebrate cell nuclei

The nuclear basket (NB) is a fibrillar scaffold anchored to the nuclear pore complex (NPC), with protein TPR constituting its structural backbone. Recently, ZC3HC1 was identified as an additional structural NB component interconnecting TPR subpopulations. However, it had remained unclear how ZC3HC1 and TPR engage each other and how the resulting ensembles are arranged relative to the NB. Here, we uncover a hierarchy of sequential interactions between distinct ZC3HC1 and TPR domains, resulting in serially arrayed TPR:ZC3HC1 ensembles appended to the NB. Projecting into the nucleus, these ensembles extend beyond the NB dimensions traditionally considered prototypic. We also find that the copy numbers of NPC/NB-associated TPR are far higher than commonly anticipated and sufficient to support this modular organizational principle. Together with sufficient ZC3HC1 copy numbers, our data define a model in which alternating recruitment of ZC3HC1 and TPR extends the NB's spatial and functional reach into the nuclear interior.