Proteolipids: small hydrophobic peptides in the field of sodium tolerance

Abstract

High soil salinity that develops in areas surrounding deserts is an increasing threat for agriculture. It has thus become an important goal for basic and applied research to identify the pathways regulating salt sensitivity in plants. Recently, major progress has led to the identification of plant genes encoding sodium transporters. The AtNHX1 protein mediates sodium detoxification by sequestration in the vacuole. The SOS1 protein has been proposed to function as Na+/H+ antiporter leading to Na+ efflux from plant cells and the AtHKT1 potassium transporter could also mediate the uptake of sodium in plants.Catherine Navarre and Andre Goffeau1xMembrane hyperpolarization and salt sensitivity induced by deletion of PMP3, a highly conserved small protein of yeast plasma membrane. Navarre, C. and Goffeau, A. EMBO J. 2000; 19: 2515–2524Crossref | PubMedSee all References1 have identified PMP3 as a new determinant of sodium tolerance in yeast. Unlike AtHKT1, AtNHX1 and SOS1, whose molecular architectures are typical for membrane transporters, PMP3 belongs to the much less characterized class of small highly hydrophobic peptides called proteolipids. In their study, Navarre and Goffeau show that, in yeast, PMP3 disruption leads to a strong increase in salt sensitivity. This phenotype is not linked to a regulation of sodium efflux pumps by PMP3 because the increase of salt sensitivity as a result of PMP3 disruption or efflux pump disruption are additive. Furthermore, PMP3 disruption restores the growth of yeast potassium transport mutants. Finally, the authors identify membrane hyperpolarization as the central mechanism underlying the increase of K+ and Na+ uptake. This effect on membrane polarization is not linked to an increase in pump activity. Therefore PMP3, a membrane protein, probably mediates or upregulates an ion leakage pathway modulating yeast membrane potential.Gene families encoding proteolipids have now been identified in bacteria, fungi, plants and animals. In plants, proteolipid genes have been found in Lophopyrum elongatum, wheat, barley and Arabidopsis. Three lines of argument suggest that they could also be involved in the determination of plant sodium sensitivity. First, they show high sequence similarity with yeast PMP3. Second, Navarre and Goffeau demonstrate that RCI2A, one of the PMP3 homologues from Arabidopsis, can complement the salt hypersensitivity phenotype of a yeast mutant with disrupted PMP3. Third, in L. elongatum, the PMP3 homologue ESI3 has been shown to be rapidly and transiently induced after exposure to high sodium concentrations, suggesting that it provides early protection against salt stress. Altogether, the data presented by Navarre and Goffeau suggest that an unexpected class of protein, the proteolipids, represent new molecular determinants for salt resistance in yeast and plants.