The majority of E3-13.7 is protected from
cell surface reduction at steady state. (A) Cartoon showing the
membrane topology of E3-13.7 proteins (Hoffman et al.,
1992a; Krajcsi et al., 1992). There are two
distinct molecular mass species, a full-length 13.7-kDa protein and an
11.3-kDa protein cleaved cotranslationally at Ala-23 (arrows). Both
species form disulfide-linked dimers at cysteine residue 31 (C31). (B
and C) Infected cells were metabolically labeled for 1 h followed
by a 3-h chase in nonradioactive medium to allow labeled proteins to
achieve steady-state distribution. Some cells were incubated with a
solution containing cysteine to reduce surface proteins with external
disulfide bonds. Cell lysates were immunoprecipitated for E3-13.7
proteins (B) or the transferrin receptor (C). Immunoprecipitates were
resolved by SDS-PAGE under reducing (+DTT) or nonreducing (−DTT)
conditions. Sizes of reduced and nonreduced protein species are
indicated to the right of each panel. E3-13.7 dimers have approximate
molecular weights of 21,000–23,400 (Hoffman et al.,
1992a). Additional molecular weight standards: phosphorylase B,
97,400; carbonic anhydrase, 31,000.