sp1 cDNA was isolated from aspen (Populus tremula) plants by immunoscreening an expression library using polyclonal antibodies against BspA protein. BspA, which is a boiling-stable protein, accumulates in aspen plants in response to water stress and abscisic acid application (Pelah et al., 1995). The sp1 cDNA was found to encode a 12.4-kD generally hydrophilic protein with a hydrophobic C terminus, which is different from the BspA protein and was termed SP1 (stable protein 1). Northern-blot analysis revealed that sp1 encodes a small mRNA (about 0.6 kb) that is expressed in aspen plants under non-stress conditions and is accumulated after salt, cold, heat, and desiccation stress, and during the recovery from stress. The SP1 detected in plants remained soluble upon boiling, migrated both as a 12.4-kD band and a much higher mass of 116 kD on a 17% (w/v) Tricine-sodium dodecyl sulfate-polyacrylamide gel. Comparative protease digestion patterns, amino acid analyses, and the N-terminal sequences of the 12.4- and 116-kD proteins revealed that SP1 is homo-oligomeric. Furthermore, gel filtration chromatography analysis indicated that SP1 exists in aspen plants as a complex, composed of 12 subunits of 12.4 kD. A large number of sequences deduced from expressed sequence tags and genomic sequences of other organisms with unknown function show high homology to SP1. Thus, SP1 may represent a new protein family. Here, we present the first report on this putative protein family: the cloning, isolation, and characterization of SP1, a stress-responsive, boiling-soluble, oligomeric protein.