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Plants lack lamin proteins but contain a class of coiled-coil proteins that serve as analogs to form a laminal structure at the nuclear periphery. These Nuclear Matrix Constituent Proteins (NMCPs) play important roles in regulating nuclear morphology and are partitioned into two distinct groups. We investigated Arabidopsis NMCPs (called CRWNs) to study the inter-relationship between the three NMCP1-type paralogs (CRWN1, 2 & 3) and the lone NMCP2-type paralog, CRWN4. An examination of crwn mutants using protein immunoblots demonstrated that CRWN4 abundance depends on the presence of the NMCP1-type proteins, particularly CRWN1. The possibility that CRWN4 is co-imported into the nucleus with NLS-bearing paralogs in the NMCP1-clade was discounted based on recovery of a crwn4-2 missense allele that disrupts a predicted nuclear localization sequence (NLS) and lowers the abundance of CRWN4 in the nucleus. Further, a screen for mutations that suppress the effects of the crwn4-2 mutation, led to the discovery of a missense allele, impa-1G146E, in one of the nine importin-α genes in the Arabidopsis genome. Our results indicate that the CRWN4 carries a functional NLS that interacts with canonic nuclear import machinery. Once imported, the level of CRWN4 within the nucleus is modulated by the abundance of NMCP1 proteins.

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