A hydrophobic proline-rich motif is involved in the intracellular targeting of temperature-induced lipocalin

Abstract

Temperature-induced lipocalins (TILs) play an essential role in the response of plants to different abiotic stresses. In agreement with their proposed role in protecting membrane lipids, TILs have been reported to be associated to cell membranes. However, TILs show an overall hydrophilic character and do not contain any signal for membrane targeting nor hydrophobic sequences that could represent transmembrane domains. Arabidopsis TIL (AtTIL) is considered the ortholog of human ApoD, a protein known to associate to membranes through a short hydrophobic loop protruding from strands 5 and 6 of the lipocalin β-barrel. An equivalent loop (referred to as HPR motif) is also present between β-strands 5 and 6 of TILs. The HPR motif, which is highly conserved among TIL proteins, extends over as short stretch of eight amino acids and contains four invariant proline residues. Subcellular localization studies have shown that TILs are targeted to a variety of cell membranes and organelles. We have also found that the HPR motif is necessary and sufficient for the intracellular targeting of TILs. Modeling studies suggest that the HPR motif may directly anchor TILs to cell membranes, favoring in this way further contact with the polar group of membrane lipids. However, some particular features of the HPR motif open the possibility that targeting of TILs to cell membranes could be mediated by interaction with other proteins. The functional analysis of the HPR motif unveils the existence of novel mechanisms involved in the intracellular targeting of proteins in plants.Electronic supplementary materialThe online version of this article (doi:10.1007/s11103-015-0326-x) contains supplementary material, which is available to authorized users.

RNAi-based functional elucidation of PtrPRP, a gene encoding a hybrid proline rich protein, in cold tolerance of Poncirus trifoliata

“…In PtPRP, proline residues account for 21% of the total amino acids of PtrPRP, allowing us to believe that PtrPRP is a PRP. Of note, the prominent abundance of proline residues at the N terminus may be possibly associated with the targeting of this protein, as the presence of hydrophobic proline-rich (HPR) motif has been suggested to be necessary and sufficient for the intracellular targeting of a temperature-induced lipocalin in Arabidopsis (AtTIL; Hernaìndez-Gras and Boronat, 2015 ). So far, PRPs have been previously categorized into five groups based on signature motifs ( Gothandam et al, 2010 ), but PtrPRP belongs to none of them.…”

The Plastid Lipocalin LCNP Is Required for Sustained Photoprotective Energy Dissipation in Arabidopsis

“…LCNP belongs to the group of true lipocalins, as it contains three SCRs (Figure 3). In the Arabidopsis genome (or other land plants), there is another true lipocalin, TIL (for temperatureinduced lipocalin; Frenette Charron et al, 2002), which locates to different cell membranes and organelles (but not the chloroplast), depending on growth conditions (Charron et al, 2005;Hernández-Gras and Boronat, 2015). TIL and LCNP play a role during abiotic stress and have overlapping functions in protecting against lipid peroxidation (Boca et al, 2014), but their mechanism of action is unknown.…”

Structure-based identification and functional characterization of an essential lipocalin in the malaria parasitePlasmodium falciparum

“…This potential membrane protective effect agrees with our observation that PfLCN not only exists as a soluble protein, but that it also associates with and integrates into membranes. Other lipocalins such as human ApoD attach to the membrane by forming a disulphide bond with a membraneembedded protein, but also use a hydrophobic loop at the opening of the binding pocket to anchor themselves into a membrane (Eichinger et al, 2007;Hernández-Gras & Boronat, 2015). PfLCN also contains a highly hydrophobic loop (residues 76-79 between β4-β5) that could be used in the same way.…”

“…Given that several lipocalins, including E. coli Blc and human ApoD, have the capability to integrate into membranes (Hernández-Gras & Boronat, 2015;Eichinger et al, 2007;Bishop et al, 1995), we next investigated the membrane association of PfLCN and performed sequential differential solubilization of lysed parasite membranes. To this aim, saponin (permeabilizes erythrocyte membrane and PVM) pellets of Lip-Kd trophozoite stage parasites were hypotonically lysed to release nonmembrane associated proteins and then sequentially treated with sodium carbonate and Triton X-100 to solubilize peripheral and integral membrane proteins, respectively.…”