Table 2

Location of Target Beads Relative to the COS Plasma Membrane after 1 h

COS transfectantDistribution of beads by location: % of total numberTotal No. of beads associated with cells
Surface*CuppedSealed§
FcγRIA  44  52   4  125
FcγRIA tailless  51  48   1   93
FcγRIA + γ chain  11  63  26   96
FcγRIIA  75   5  20   72
COS transfectantDistribution of beads by location: % of total numberTotal No. of beads associated with cells
Surface*CuppedSealed§
FcγRIA  44  52   4  125
FcγRIA tailless  51  48   1   93
FcγRIA + γ chain  11  63  26   96
FcγRIIA  75   5  20   72

COS cell transfectants were incubated with IgG-coated latex beads at 37°C for 1 h and were then fixed and processed for electron microscopy. The position of individual beads was determined relative to the labeled COS cell plasma membrane. The combined data from two experiments sort into three categories that define the position of the bead:

*

Surface: the bead is bound to the external membrane surface with little or no pseudopod advancement. The measured arc of contact between the COS cell membrane and bead was 90° ± 30° for 30 beads examined in this category.

Cupped: the bead is enveloped by pseudopods covering 50% or more of the bead circumference. The measured arc of contact between pseudopods and bead targets was 280° ± 60° for 30 beads examined in this category.

§

Sealed: the bead is sealed inside the cell as shown by a lack of external membrane staining around the bead.

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