We report the finding of a secretory heat shock protein, HSP150, of Saccharomyces cerevisiae, and the characterization of the gene coding for it. HSP150 is constitutively expressed, extensively O-glycosylated, and secreted efficiently to the growth medium. When cells grown at 25 degrees C were shifted to 37 degrees C, a 7-fold increase in the level of HSP150 was observed within 1 hr. The HSP150 gene encodes a primary translation product of 412 amino acids. Direct amino acid sequencing of the mature secreted protein showed that an N-terminal sequence of 18 amino acids is removed, and a KEX2 protease-specific site is cleaved to yield two subunits of 53 and 341 amino acids, which remain noncovalently associated during secretion. The larger subunit is highly repetitive, containing 11 tandem repeats of a 19-amino acid sequence. Northern blot hybridization analysis showed a substantial increase in HSP150 mRNA level after heat shock. The upstream flanking region of the gene contains several heat shock element-like sequences. Disruption of HSP150 did not lead to inviability or significant effects on growth rate, mating, or thermotolerance. However, heat-regulated antigenic homologs of HSP150 were found in divergent yeasts such as Schizosaccharomyces pombe.