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Department of Medicine, Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis,
Missouri 63110
Connexins are gap junction proteins that
form aqueous channels to interconnect adjacent cells.
Rat osteoblasts express connexin43 (Cx43), which
forms functional gap junctions at the cell surface. We
have found that ROS 17/2.8 osteosarcoma cells, UMR
106-01 osteosarcoma cells, and primary rat calvarial osteoblastic cells also express another gap junction protein, Cx46. Cx46 is a major component of plasma membrane gap junctions in lens. In contrast, Cx46 expressed by osteoblastic cells was predominantly localized to an
intracellular perinuclear compartment, which appeared
to be an aspect of the TGN as determined by immunofluorescence colocalization. Hela cells transfected with
rat Cx46 cDNA (Hela/Cx46) assembled Cx46 into functional gap junction channels at the cell surface. Both rat lens and Hela/Cx46 cells expressed 53-kD (nonphosphorylated) and 68-kD (phosphorylated) forms of
Cx46; however, only the 53-kD form was produced by
osteoblasts. To examine connexin assembly, monomers
were resolved from oligomers by sucrose gradient velocity sedimentation analysis of 1% Triton X-100-solubilized extracts. While Cx43 was assembled into multimeric complexes, ROS cells contained only the
monomer form of Cx46. In contrast, Cx46 expressed by
rat lens and Hela/Cx46 cells was assembled into multimers. These studies suggest that assembly and cell surface expression of two closely related connexins were
differentially regulated in the same cell. Furthermore,
oligomerization may be required for connexin transport from the TGN to the cell surface.
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