Polyubiquitination of PS1 protein in HEK293 cells: effect of Z-IE(Ot-Bu)A-Leucinal. Stably transfected HEK293 cells expressing wtPS1 were preincubated with 25 µM of Z-IE(Ot-Bu)A-Leucinal during the indicated time periods, then cells were scraped, and lysed in RIPA buffer. wtPS1 was immunoprecipitated with αPS1Loop antibody (1/1000e dilution) as described in Materials and Methods. Immunoprecipitates were rinsed with RIPA buffer then electrophoresed on an 8% SDS-PAGE then Western blotted as described in Materials and Methods. Ubiquitin-like immunoreactivity was revealed with a monoclonal anti-ubiquitin antibody (Ubi-1). Control (C) corresponds to cells preincubated for 24 hr with the adequate DMSO concentration.