(A) BACE1WT, BACE1Ala, and BACE1Gln cells were labelled with a mixture of radioactive methionine/cysteine for 10 min and then chased for 2 h. Pro-BACE1 was immunoprecipitated from total cell lysates with a specific antibody against the pro-domain region of BACE1. Unbound BACE1 (without the pro-domain) was then immunoprecipitated with an antibody against the C-terminal domain of BACE1. Immunoprecipitates were analysed by SDS/PAGE and autoradiography. (B) BACE1WT, BACE1Ala and BACE1Gln cells were labelled with a mixture of radioactive methionine/cysteine for 10 min and then chased for 1.5 h. Cell surface proteins were biotinylated, separated with Immobilized Streptavidin and eluted by lowering the pH. BACE1 was then immunoprecipitated as above and analysed by reducing SDS/PAGE and autoradiography. The typical migration of cell-surface BACE1 on reducing SDS/PAGE is shown in Supplementary Figures 1A and 1B at http://www.BiochemJ.org/bj/407/bj4070383add.htm. Results are expressed as a percentage of total newly synthesized BACE1. *P<0.005; n=4. (C and D) BACE1WT, BACE1Ala and BACE1Gln cells were labelled with a mixture of radioactive methionine/cysteine for 30 min. and then chased for 8, 16 and 24 h. BACE1 was immunoprecipitated from total cell lysates and analysed by SDS/PAGE and autoradiography. The half-life of BACE1 from five different readings is shown in (C), whereas a representative radiogram is shown in (D). (E) Cell lysates from BACE1WT, BACE1Ala and BACE1Gln cells were separated either on a 7% Tris/Acetate (TA) or on a 4–12% Bis/Tris (BT) SDS/PAGE system, blotted on to a PVDF membrane, and probed with anti-BACE1 antibodies. Both immature (im. BACE1) and mature (m. BACE1) BACE1 are indicated. (F) Autoradiography of BACE1 immunoprecipitated from stably transfected cells immediately after a 30 min period of labelling with [35S]methionine/cysteine (top panel). Samples were separated on a 4–12% Bis/Tris SDS/PAGE system. BACE1 and GAPDH (glyceraldehyde-3-phosphate dehydrogenase; control lane) mRNA levels were quantified after total RNA extraction and RT (reverse transcriptase)-PCR (bottom panel). IP, immunoprecipitate; WT, wild-type.