Heavy Metal-Associated Isoprenylated Plant Proteins (HIPPs) at Plasmodesmata: Exploring the Link between Localization and Function

Abstract

Heavy metal-associated isoprenylated plant proteins (HIPPs) are a metallochaperone-like protein family comprising a combination of structural features unique to vascular plants. HIPPs possess both one or two heavy metal-binding domains and an isoprenylation site, facilitating a posttranslational protein lipid modification. Recent work has characterized individual HIPPs across numerous different species and provided evidence for varied functionalities. Interestingly, a significant number of HIPPs have been identified in proteomes of plasmodesmata (PD)-nanochannels mediating symplastic connectivity within plant tissues that play pivotal roles in intercellular communication during plant development as well as responses to biotic and abiotic stress. As characterized functions of many HIPPs are linked to stress responses, plasmodesmal HIPP proteins are potentially interesting candidate components of signaling events at or for the regulation of PD. Here, we review what is known about PD-localized HIPP proteins specifically, and how the structure and function of HIPPs more generally could link to known properties and regulation of PD.

Keywords: HIPP; abiotic and biotic stress; cytokinin; heavy metal-associated plant proteins; metallochaperone; plasmodesmata; prenylation.

Figure 1

HIPP proteins and their PD association. (A) Schematic of the domain structure with one (OsHIPP19: Q01IL6) or two (AtHIPP7: Q9C5D3) HMA domains. The HMA domain (magenta) and the prenylation site (grey) are annotated with their respective motifs. (B) Crystal structure of OsHIPP19 HMA domain. Cysteine residues of the CXXC motif are labeled and the characteristic βαββαβ fold is shown with beta strands (green) and alpha helices (magenta). Image from the RCSB PDB (RCSB.org) of PDB ID 7B1I [4,31,32]. (C) Confocal microscopy of Arabidopsis leaf epidermal cells stably expressing UBQ10:GFP-HIPP7 (C) and UBQ10:GFP-HIPP1 (D). Scale bars, 10 µm. Adapted with permission from Ref. [33]. Copyright 2021, Cell Press (Elsevier).