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A Novel Polyubiquitin Structure in Cercozoa and Foraminifera: Evidence for a New Eukaryotic Supergroup

Summary: A Novel Polyubiquitin Structure in Cercozoa and Foraminifera:
Evidence for a New Eukaryotic Supergroup
John M. Archibald,* David Longet, Jan Pawlowski, and Patrick J. Keeling*
*Canadian Institute for Advanced Research, Department of Botany, University of British Columbia, Vancouver, British Columbia,
Canada; and De´partement de Zoologie et Biologie Animale, University of Geneva, Geneva, Switzerland
Ubiquitin is a 76 amino acid protein with a remarkable degree of evolutionary conservation. Ubiquitin plays an essential
role in a large number of eukaryotic cellular processes by targeting proteins for proteasome-mediated degradation. Most
ubiquitin genes are found as head-to-tail polymers whose products are posttranslationally processed to ubiquitin
monomers. We have characterized polyubuiquitin genes from the photosynthetic amoeboflagellate Chlorarachnion sp.
CCMP 621 (also known as Bigelowiella natans) and found that they deviate from the canonical polyubiquitin structure in
having an amino acid insertion at the junction between each monomer, suggesting that polyubiquitin processing in this
organism is unique among eukaryotes. The gene structure indicates that processing likely cleaves monomers at the amino
terminus of the insertion. We examined the phylogenetic distribution of the insertion by sequencing polyubiquitin genes
from several other eukaryotic groups and found it to be confined to Cercozoa (including Chlorarachnion, Lotharella,
Cercomonas, and Euglypha) and Foraminifera (including Reticulomyxa and Haynesina). This character strongly suggests
that Cercozoa and Foraminifera are close relatives and form a new ``supergroup'' of eukaryotes.
Ubiquitin-mediated protein degradation is an impor-
tant biochemical process in eukaryotic cells. Through its
covalent attachment to other proteins, ubiquitin plays


Source: Archibald, John - Department of Biochemistry and Molecular Biology, Dalhousie University
Keeling, Patrick - Department of Botany, University of British Columbia


Collections: Biology and Medicine; Environmental Sciences and Ecology