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First published online November 5, 2008
doi: 10.1242/10.1242/jcs.037507

Journal of Cell Science 121, 3683-3692 (2008)
Published by The Company of Biologists 2008
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Novel functions of the CD34 family

Julie S. Nielsen1 and Kelly M. McNagny2,*

1 Trev and Joyce Deeley Research Centre, British Columbia Cancer Agency, 2410 Lee Avenue, Victoria, BC, Canada V8R 6V5
2 The Biomedical Research Centre, 2222 Health Sciences Mall, University of British Columbia, Vancouver, BC, Canada V6T 1Z3


Figure 1
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  		Fig. 1. Protein structures, genomic organization and splicing of the CD34 family. (A) Schematic of protein structures. CD34, podocalyxin and endoglycan each have an extensively O-glycosylated (horizontal lines) and sialylated (horizontal lines with arrowheads) serine-, threonine- and proline-rich extracellular mucin domain (green), putative sites of N-glycosylation (lines with circles), a cysteine-containing globular domain (dark blue) and a juxtamembrane stalk region (yellow). Their single-pass transmembrane domains (light blue) are followed by short cytoplasmic tails (red) containing putative phosphorylation sites and C-terminal PDZ-domain docking sites (DTEL or DTHL). The extracellular unpaired cysteine residue of endoglycan can facilitate homodimerization; endoglycan also contains an unusual polyglutamic-acid-rich extracellular domain (pink box). (B) Genomic organizations. Each protein is encoded by eight exons, with individual exons encoding the corresponding domain in each protein. (C) Alternative splicing. CD34 and podocalyxin display identical patterns of alternative splicing, and can exist as truncated versions that lack most of the cytoplasmic tail.

 

Figure 2
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  		Fig. 2. Proposed functions of CD34-family proteins: promoting and blocking adhesion. (A) L-selectin expressed on naive lymphocytes recognizes HEV-specific glycosylation motifs on CD34-family members and mediates cell adhesion. (B) The bulky, negatively charged extracellular domains of CD34 family members block cell adhesion by charge repulsion and steric hindrance; this prevents interaction of integrins with their ligands.

 

Figure 3
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  		Fig. 3. Levels of podocalyxin expression might explain its apparently contradictory roles in cell adhesion. (A) Membrane-protein-segregation model. Low levels of podocalyxin establish apical domains and force integrins to the basal surface of cells, thereby enhancing cell adhesion. (B) High levels of podocalyxin strongly induce microvillus formation. Dramatic relocalization of actin to the apical membrane to support formation of microvilli might deplete basolateral actin, thereby disrupting integrin-mediated adhesion.

 

Figure 4
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  		Fig. 4. Possible functions of CD34-family proteins: chemokine-mediated trafficking and regulation of asymmetric cell division. (A) Model of podocalyxin-dependent chemotaxis. (B) Two proposed mechanisms by which podocalyxin might facilitate asymmetric cell division. Segregation of podocalyxin to the apical surface might enable the interaction of adhesion molecules with the stem-cell niche; after division, only the cell that remains in contact with the niche would receive signals to maintain pluripotency. Alternatively, podocalyxin- and NHERF1-dependent segregation of cell-fate determinants might instruct the cell to divide asymmetrically.

 

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© The Company of Biologists Ltd 2008