Archibald J M, Cavalier-Smith T, Maier U, Douglas S
Department of Biochemistry and Molecular Biology, Dalhousie University, Halifax, Nova Scotia, B3H 4H7, Canada.
J Mol Evol. 2001 Jun;52(6):490-501. doi: 10.1007/s002390010179.
Molecular chaperones mediate the correct folding of nascent or denatured proteins and are found in both the organelles and cytoplasm of eukaryotic cells. Cryptomonad algae are unusual in possessing an extra cytoplasmic compartment (the periplastid space), the result of having engulfed and retained a photosynthetic eukaryote. Within the periplastid space is a diminutive nucleus (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 from the cryptomonad, Guillardia theta. Here we analyse G. theta nucleomorph genes for members of the cytosolic HSP70 and HSP90 families of molecular chaperones, a heat shock transcription factor (HSF), and all eight subunits of the group II chaperonin, CCT. These are presumably all active in the periplastid 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 co-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 genome and targeted back into the periplastid space. Endoplasmic reticulum (ER) homologs of HSP70 and HSP90 are also not present. Striking differences in the degree of conservation of the various nucleomorph-encoded molecular chaperones were observed. While the G. theta HSP70 and HSP90 homologs are well conserved, each of the eight CCT subunits (alpha, beta, gamma, delta, epsilon, eta, theta, and zeta) is remarkably divergent. Such differences are likely evidence for reduced/different functional constraints on the various molecular chaperones functioning in the periplastid space.
分子伴侣介导新生或变性蛋白质的正确折叠，存在于真核细胞的细胞器和细胞质中。隐藻门藻类不同寻常之处在于拥有一个额外的细胞质区室（周质体空间），这是吞噬并保留了一个光合真核生物的结果。周质体空间内有一个微小的细胞核（核质体），它主要编码自身表达所需的基因以及质体所需的一些基因。已报道来自隐藻Guillardia theta的两种质体编码的伴侣蛋白（GroEL和DnaK）以及一种核质体编码的伴侣蛋白（Cpn60）。在这里，我们分析了Guillardia theta核质体基因中的胞质HSP70和HSP90分子伴侣家族成员、一个热休克转录因子（HSF）以及伴侣蛋白CCT的所有八个亚基。这些蛋白可能都在周质体空间中发挥作用，协助细胞所需多肽的成熟；我们还提出它们在一个假定的残余有丝分裂装置的结构和组装中也起着核心作用。奇怪的是，在核质体基因组中未检测到HSP70、HSP90或CCT的共伴侣蛋白基因；它们要么不需要，要么由宿主核基因组编码并靶向回到周质体空间。内质网（ER）的HSP70和HSP90同源物也不存在。观察到各种核质体编码的分子伴侣在保守程度上存在显著差异。虽然Guillardia theta的HSP70和HSP90同源物保守性良好，但CCT的八个亚基（α、β、γ、δ、ε、η、θ和ζ）中的每一个都有显著差异。这种差异可能是周质体空间中各种分子伴侣功能限制减少/不同化的证据。
