LOW-MOLECULAR-WEIGHT HEAT-SHOCK PROTEINS IN A DESERT FISH (POECILIOPSIS-LUCIDA) - HOMOLOGS OF HUMAN HSP27 AND XENOPUS HSP30

The heat shock response of a fish which inhabits a highly stressful en vironment (Poeciliopsis lucida, a minnow from river systems of the Son oran desert in northwestern Mexico) was investigated. Cells derived fr om this fish exhibited a typical heat shock response when exposed to e levated temperature, synthesizing high levels of 90 kDa, 70 kDa, and 3 0 kDa heat shock proteins (Hsp90, Hsp70, and Hsp30), as well as lower amounts of other heat shock proteins. Additional small heat shock prot eins (sHSPs), including Hsp27, were induced after a prolonged heat sho ck at a time when synthesis of Hsp70 and Hsp30 was decreasing. Charact erization of cDNA clones for hsp27 and hsp30 revealed that both are me mbers of the alpha-crystallin/sHSP superfamily but belong to separate lineages within this gene family. The multiple isoforms of P. lucida H sp30 appear to be members of a multigene family and are most closely r elated to salmon and Xenopus Hsp30s. In contrast, Hsp27 is highly simi lar to mammalian and avian sHSPs; it was synthesized as three isoforms which represented differentially phosphorylated forms of a single pol ypeptide. In Poeciliopsis, the various sHSPs may each perform a subset of the roles attributed to mammalian sHSPs. The conservation of phosp horylation sites in Hsp27 may indicate an involvement in signal transd uction to the actin cytoskeleton. The hsp30 genes appear to have diver ged more rapidly than the corresponding hsp27 genes; the various membe rs of the Hsp30 family may function as molecular chaperones and, in th is role, may be less evolutionarily constrained. Finally, the presence of these two classes of sHSP in a single taxon indicates that these t wo lineages arose by gene duplication early in the evolution of verteb rates and raises questions about the fate of homologs of Hsp30 in mamm als and of Hsp27 in Xenopus.