Phosphatidylserine exposure promotes increased adhesion in Dictyostelium Copine A mutants

The phospholipid phosphatidylserine (PS) is a key signaling molecule and
binding partner for many intracellular proteins. PS is normally found on
the inner surface of the cell membrane, but PS can be flipped to the outer
surface in a process called PS exposure. PS exposure is important in many
cell functions yet the mechanisms that control PS exposure have not been
extensively studied. Copines (Cpn), found in most eukaryotic organisms,
make up a family of calcium-dependent phospholipid binding proteins. In
Dictyostelium, which has six copine genes, CpnA strongly binds to PS and
translocates from the cytosol to the plasma membrane in response to a rise
in calcium.  Cells lacking the cpnA gene (cpnA- ) have defects in
adhesion, chemotaxis, membrane trafficking and cytokinesis. In this study
we used both flow cytometry and fluorescent microscopy to show that cpnA-
cells have increased adhesion to beads and bacteria and that the increased
adhesion was not due to changes in the actin cytoskeleton or cell surface
proteins. We found that cpnA- cells bound higher amounts of Annexin V, a
PS binding protein, than parental cells and showed that unlabeled Annexin
V reduced the increased cell adhesion property of cpnA- cells. We also
found that cpnA- cells were more sensitive to Polybia-MP1, which binds to
external PS and induces cell lysis. Overall, this suggests that cpnA-
cells have increased PS exposure and this property contributes to the
increased cell adhesion of cpnA- cells. We conclude that CpnA has a role
in the regulation of plasma membrane lipid composition and may act as a
negative regulator of PS exposure.