Previously, the growth of Arabidopsis hit1-1 (heat-intolerant) mutant was found to be inhibited by both heat and water stress (Wu et al. in J Plant Physiol 157:543-547, 2000). In order to determine the genetic mutation underlying the hit1-1 phenotype, map-based cloning of HIT1 gene was conducted. Transformation of the hit1-1 mutant with a HIT1 cDNA clone reverts the mutant to the heat tolerance phenotype, confirming the identity of HIT1. Sequence analysis revealed the HIT1 gene encodes a protein of 829 amino acid residues and is homologous to yeast (Saccharomyces cerevisiae) Vps53p protein. The yeast Vps53p protein has been shown to be a tethering factor that associates with Vps52p and Vps54p in a complex formation involved in the retrograde trafficking of vesicles to the late Golgi. An Arabidopsis homolog of yeast Vps52p has previously been identified and mutation of either the homolog or HIT1 by T-DNA insertion resulted in a male-specific transmission defect. The growth of yeast vps53Delta null mutant also shows reduced thermotolerance, and expression of HIT1 in this mutant can partially complement the defect, supporting the possibility of a conserved biological function for Vps53p and HIT1. Collectively, the hit1-1 is the first mutant in higher plant linking a homolog of the vesicle tethering factor to both heat and osmotic stress tolerance.