Overexpression of Rpn1p inhibits degradation of N-end rule substrates.
A UPR-based pulse–chase assay was used (see Results).
S. cerevisiae JD52 (UBR1) cells
coexpressing either DHFR-HA-UbR48-Leu-βgal
(A) or DHFR-HA-UbR48-His-βgal
(B), and either Rpn1p (expressed from its own promoter
in pRS425) or pRS425 alone were labeled with
[35S]methionine for 5 min at 30°C, followed by a chase
for 0, 10, and 30 min. Cell extracts were immunoprecipitated with both
anti-βgal and anti-HA antibodies, followed by SDS/12% PAGE,
autoradiography, and quantitation using PhosphorImager (Molecular
Dynamics). (C) His-βgal and Leu-βgal decay curves
calculated from the UPR-based data in A and
B as described (28, 32). The bands of X-βgals and the
reference protein DHFR-HA-UbR48 (“ref”) are
indicated.