Molecular chaperones encoded by a reduced nucleus: The cryptomonad nucleomorph

Molecular chaperones mediate the correct folding of nascent or denatured pr oteins and are found in both the organelles and cytoplasm of eukaryotic cel ls. Cryptomonad algae are unusual in possessing an extra cytoplasmic compar tment (the periplastid space), the result of having engulfed and retained a photosynthetic eukaryote. Within the periplastid space is a diminutive nuc leus (the nucleomorph) that encodes mostly genes for its own expression as well as a few needed by the plastid. Two plastid-encoded chaperones (GroEL and DnaK) and a nucleomorph-encoded chaperone (Cpn60) have been reported fr om the cryptomonad, Guillardia theta. Here we analyse G. theta nucleomorph genes for members of the cytosolic HSP70 and HSP90 families of molecular ch aperones, a heat shock transcription factor (HSF), and ail eight subunits o f the group II chaperonin, CCT. These are presumably all active in the peri plastid space, assisting in the maturation of polypeptides required by the cell; we propose a central role for them also in the structure and assembly of a putative relict mitotic apparatus. Curiously, none of the genes for c o-chaperones of HSP70, HSP90, or CCT have been detected in the nucleomorph genome; they are either not needed or are encoded in the host nuclear genom e and targeted back into the periplastid space. Endoplasmic reticulum (ER) homologs of HSP70 and HSP90 are also not present. Striking differences in t he degree of conservation of the various nucleomorph-encoded molecular chap erones were observed. While the G. theta HSP70 and HSP90 homologs are well conserved, each of the eight CCT subunits (alpha, beta, gamma, delta, epsil on, eta, theta, and xi) is remarkably divergent. Such differences are likel y evidence for reduced/ different functional constraints on the various mol ecular chaperones functioning in the periplastid space.