Table 1.

Quantitation of CD8-K and CD8-E19 at ER exit sites

Cell expressing	μm ER	No. of protrusions	Labeled protrusions (% total)	Gold particles in ER (% total)	Gold particles in protrusions (% ER-total)
CD8-K	219	77	54 (70)	3240 (89)	78 (2.4)
CD8-E19	189	73	28 (38)	3028 (70)	33 (1.1)

Parallel cultures of HuH-7 cells were transfected with CD8-K and CD8-E19 expression plasmids and processed for cryosectioning and immunogold labeling as detailed in the MATERIALS AND METHODS section. Transfection conditions were optimized to obtain roughly the same expression level of the two reporters. Fifteen low magnification micrographs of different expressing cells were taken at random for each transfection. Different enlargements of the micrographs were used to measure the following: the total number of gold particles, the number of gold particles in the ER, the total length of ER membranes, the total number of protrusions (see Figure 5), the number of gold-labeled protrusions, the total number of gold particles in protrusion. As judged by tracing a line to connect the ER membrane at the basis of the protrusion, only gold particles within the space of a protrusion were counted.