Xoom is maternally stored and functions as a transmembrane protein for gastrulation movement in Xenopus embryos

Abstract

Xoom has been identified as a novel gene that plays an important role in gastrulation of Xenopus laevis embryo. Although Xoom is actively transcribed during oogenesis, distribution and function of its translation product have not yet been clarified. In the present study, the polyclonal antibody raised against Xoom was generated to investigate a behavior of Xoom protein. Anti‐Xoom antibodies revealed that there are two forms of Xoom protein in Xenopus embryos: (i) a 45 kDa soluble cytoplasmic form; and (ii) a 44 kDa membrane‐associated form. Two forms of Xoom protein were ubiquitously detected from unfertilized egg to tadpole stage, with a qualitative peak during blastula and gastrula stages. Immunohistochemical examination showed that Xoom protein is maternally stored in the animal subcortical layer and divided into presumptive ectodermal cells during cleavage stages. Enzymatic digestion of membrane protein and immunologic detection of Xoom showed that Xoom exists as a membrane‐associated protein. To examine a function of Xoom protein, anti‐Xoom antibodies were injected into blastocoele of stage 7 blastula embryo. Anti‐Xoom antibodies caused gastrulation defect in a dose‐ dependent manner. These results suggest that maternally prepared Xoom protein is involved in gastrulation movement on ectodermal cells.

hRpn13/ADRM1/GP110 is a novel proteasome subunit that binds the deubiquitinating enzyme, UCH37

“…Many proteasome subunits are essential for cell viability, but most isopeptidases are disposable for cells (Guterman and Glickman, 2004). The homolog of UCH37 in Drosophila , dUCH37, does not seem to be required for proteasome activity in S2 cells (Wojcik and DeMartino, 2002), but the homolog of hRpn13 in frogs, Xoom, is required for embryonic development (Hasegawa et al , 2001). To determine the importance of hRpn13 in survival of mammalian cells, HeLa cells were transiently transfected with the Myc/His6‐tagged full‐length, the N‐terminal half, or the C‐terminal half of hRpn13 and were then analyzed by immunostaining (Figure 7A).…”

“…Therefore, hRpn13 also seems to be important in assembly of the 19S complex or its function at some UCH37/UCH2‐independent steps. In frogs, the homolog of hRpn13, Xoom, is required for embryonic development (Hasegawa et al , 2001), which presumably reflects a critical role for protein degradation or for UCH37 during frog development. Knockdown of hRpn13 in 293T cells, though incomplete, reduced overall degradation of short‐lived proteins reproducibly, and this two‐fold inhibition caused a large accumulation of the rapidly degraded model substrate Ub‐R‐GFP, which is degraded by the proteasome after ubiquitination by the ‘N‐end rule’ ligase E3α/Ubr1 (Varshavsky, 1997).…”

“…Also, the 26S particles in lower eukaryotes seem to contain unique subunits, such as Daq1/Rpn13, a 156 amino‐acid subunit of the 19S in budding yeast that appears to be important in the degradation of certain model substrates (Verma et al , 2000). No similar subunit has been reported in mammalian proteasomes, although interestingly, Rpn13 is highly homologous to the N‐terminal half of Xoom, a 404 amino‐acid protein required for the embryonic development of frogs (Hasegawa et al , 2001).…”

FoxI1e activates ectoderm formation and controls cell position in theXenopusblastula

“…Cell mixing across this boundary is prevented by expression of surface proteins that control cell position by differential adhesion. Many elements of this model require further testing, including the functional relationship between FoxI1e and other factors known to be important in ectoderm differentiation such as ectodermin (Dupont et al, 2005) Xoom (Hasegawa et al, 2001) and Xlim5 (Houston and Wylie, 2003), as well as the epidermal proteins AP-2 (Luo et al, 2002) and grainyhead-like1 (Tao et al, 2005), and the many early CNS factors including Sox2 and Sox3. It is also likely that there will be signaling pathways in addition to nodals that control the spatial and temporal expression of FoxI1e, and we know little detail concerning its downstream targets.…”

Regulators of the Proteasome Pathway, Uch37 and Rpn13, Play Distinct Roles in Mouse Development

“…However, one study found that overexpression of the C-terminal domain of Rpn13 that modifies Uch37 function, led to cell death in the 293T cell line [10]. To our knowledge, only Rpn13 was studied in a higher organism, the frog, where the Rpn13 homologue, Xoom was indispensable for survival of frog embryos [17].…”