WPP-domain proteins mimic the activity of the HSC70-1 chaperone in preventing mistargeting of RanGAP1-anchoring protein WIT1.

Arabidopsis (Arabidopsis thaliana) tryptophan-proline-proline (WPP)-domain proteins, WPP1 and WPP2, are plant-unique, nuclear envelope-associated proteins of unknown function. They have sequence similarity to the nuclear envelope-targeting domain of plant RanGAP1, the GTPase activating protein of the small GTPase Ran. WPP domain-interacting tail-anchored protein 1 (WIT1) and WIT2 are two Arabidopsis proteins containing a coiled-coil domain and a C-terminal predicted transmembrane domain. They are required for RanGAP1 association with the nuclear envelope in root tips. Here, we show that WIT1 also binds WPP1 and WPP2 in planta, we identify the chaperone heat shock cognate protein 70-1 (HSC70-1) as in vivo interaction partner of WPP1 and WPP2, and we show that HSC70-1 interacts in planta with WIT1. WIT1 and green fluorescent protein (GFP)-WIT1 are targeted to the nuclear envelope in Arabidopsis. In contrast, GFP-WIT1 forms large cytoplasmic aggregates when overexpressed transiently in Nicotiana benthamiana leaf epidermis cells. Coexpression of HSC70-1 significantly reduces GFP-WIT1 aggregation and permits association of most GFP-WIT1 with the nuclear envelope. Significantly, WPP1 and WPP2 show the same activity. A WPP1 mutant with reduced affinity for GFP-WIT1 fails to decrease its aggregation. While the WPP-domain proteins act on a region of WIT1 containing the coiled-coil domain, HSC70-1 additionally acts on the C-terminal transmembrane domain. Taken together, our data suggest that both HSC70-1 and the WPP-domain proteins play a role in facilitating WIT1 nuclear envelope targeting, which is, to our knowledge, the first described in planta activity for the WPP-domain proteins.