The protective action of chaperone-like activity of HtrA protease against protein aggregation was studied. High levels of proteolytically inactive HtrAS210A (active center serine replaced by alanine) suppressed the temperature-sensitive phenotype of the htrA mutants. The ability of HtrAS210A to alleviate the lethality of htrA bacteria at high temperatures correlated well with the observed decrease of cellular level of large protein aggregates in cells overproducing HtrAS210A. The in vitro experiments proved that HtrA was very efficient in inhibiting the unfolded substrate (lysozyme) aggregation over a wide range of temperatures (30-45 degrees C). HtrA was able to bind to the denatured polypeptides and as a consequence limited their ability to form large aggregates. Our results suggest that HtrA may protect the bacterial cells from deleterious effects of heat shock not only by degrading the damaged proteins but by combination of the proteolytic and chaperoning activities.