Tobacco and Arabidiopsis SLT1 mediate salt tolerance of yeast

A tobacco cDNA (NtSLT1, for Nicotiana tabacum sodium- and lithium-tolerant) was isolated by functional complementation of the salt-sensitive phenotype of a calcineurin (CaN)-deficient yeast mutant (cnb Delta, regulatory subun it null). CaN is a Ca2+/calmodulin-dependent type 2B protein phosphatase th at regulates Na+ homeostasis in yeast. This phosphatase modulates plasma me mbrane K+/Na+ selectivity through the activation of high-affinity K+ transp ort, and increaseses extracellular Na+ efflux by activation and transcripti onal induction of the Na+/Li(+)translocating P-type ATPase encoded by ENA1. Expression of N-terminally truncated NtSLT1 (Met-304), but not full-length protein, suppressed salt sensitivity of cnb1. Truncated NtSLT1 also increa sed salt tolerance of wild-type yeast, indicating functional sufficiency. N tSLT1 encodes a protein of yet unknown function but experimentation in yeas t confirms it as a salt tolerance determinant. The Arabidopsis thaliana ort hologue, AtSLT1, also suppressed salt sensitivity of cnb Delta but only whe n expressed without the N-terminus (Met-301), suggesting that this region o f the proteins from these evolutionarily diverse plant species contains an autoinhibitory domain. NtSLT1 enhanced transcription of the CaN-dependent E NA1 gene promoter and compensated the salt sensitivity of a mutant deficien t in TCN1 - a transcription factor that is activated by CaN and then induce s ENA1 expression. NtSLT1 partially suppressed the salt sensitivity of ena1 -4 indicating that NtSLT1 has both ENA-dependent and independent functions. NtSLT1 suppressed spk1 hal4 (SPK1/HAL4 which encodes a serine-threonine ki nase that regulates TRK1-2 transporters to have high K+/Na+ selectivity) bu t not ena1-4 trk1-2 implicating the ENA-independent function to be through TRK1-2. Together, these results implicate SLT1 as a signal regulatory molec ule that mediates salt tolerance by modulating Na+ homeostasis.