Search for the cellular functions of plant Hsp100/Clp family proteins

Hsp100/Clp family of proteins is ubiquitously distributed in living systems . Detailed work carried out in bacterial and yeast cells has shown that reg ulatory members of the Clp family (mainly ClpA, ClpB, and ClpC), together w ith the catalytic subunit (mainly ClpP), comprise an ATP-dependent two-comp onent proteolytic system. Members of the Hsp 100/Clp protein family are not only involved in the regulation of energy-dependent protein hydrolysis but also function as molecular chaperones. However, the biochemical/physiologi cal role(s) of the Hsp100/Clp protein family in higher plants has yet to be elucidated. Recently, this protein family has been implicated in plant str ess responses: the hor1 mutant of Arabidopsis thaliana, which has mutation in hsp101 gene, and is defective in tolerance to high temperature (S.-W. Ho ng and E. Vierling, 2000, Proc Natl Acad Sci USA, 97 (8), 4392-4397) and th e transgenic Arabidopsis thaliana plants overexpressing ArHsp101 gene exhib it high temperature tolerance (C. Quietsch et al., 2000, Plant Cell, 12, 47 9-492). Furthermore, the Hsp101 protein is involved in the translational re gulation of cellular mRNAs and one such candidate has been identified as th e photosynthetic electron transport gene Ferredoxin 1 mRNA (J. Ling et al., 2000, Plant Cell, 12, 1213-1227). We present what is known about the bacte rial, yeast, and plant Hsp 100/Clp proteins, discuss their possible relatio nship, and, more importantly, examine the cellular roles that this importan t family of proteins plays in plants.