Small Heat Shock Proteins Protect Electron Transport in Chloroplasts and Mitochondria During Stress

Evidence indicates that small heat-shock proteins (Hsps) areinvolved in stress tolerance, but the specific cell componentsor functions that small Hsps protect or repair are mostly unidentified.We recently showed that the chloroplast small Hsps of higherplants (1) are produced in response to many environmental stresses(e.g., heat, oxidative, and high-light stress); and (2) protect(but do not repair) photosynthetic electron transport in vitroduring stress, specifically by interacting with the oxygen-evolving-complexproteins of Photosystem II (PSII) within the thylakoid lumen.However, in vivo evidence of the importance of these Hsps tophotosynthetic stress tolerance is lacking. Here we report positiverelationships between chloroplast small Hsp production and PSIIthermotolerance in (1) a heattolerant genotype of Agrostis palustris(bentgrass) and a heat sensitive genotype which lacks one ormore chloroplast small Hsps produced by the tolerant genotype;(2) ecotypes of Chenopodium album (lambs quarters) from thenorthern vs. southern U.S. (New York vs. Georgia); and (3) nineLycopersicon (tomato) cultivars/species differing in heat tolerance.These in vivo results are consistent with our previous in vitroobservations and indicate that genetic variation in productionof the chloroplast small Hsp is an important determinant ofphotosynthetic and, thereby, whole-plant thermotolerance. Recently,we showed that the mitochondrial small Hsp of plants protectsrespiratory (specifically Complex I) electron transport in vitroduring heat stress, and here we present evidence for previouslyunidentified small Hsps in mitochondria of mammal (rat) cellswhich also protect Complex I during heat stress. These resultssuggest that the mitochondrial small Hsps, like the small chloroplastHsps, are general stress proteins that contribute significantlyto cell and organismal stress tolerance.