Hierarchical TPR:ZC3HC1 interactions drive stepwise assembly of nuclear basket extensions projecting into vertebrate cell nuclei

Abstract

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 TPR:ZC3HC1 ensembles are arranged relative to the NB. Here, we uncover a hierarchy of sequential interactions between distinct ZC3HC1 and TPR domains, resulting in NB-appended ZC3HC1:TPR ensembles that extend beyond the NB dimensions traditionally considered prototypic. Consistent with a modular organisational principle, we furthermore find copy numbers of NPC/NB-associated TPR to be far higher than commonly anticipated. 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.