As used to dry them prior to resuspension in 5 (v/v) formic acid. Peptide concentrations were determined using the CBQCA assay after 25-fold dilution of peptide samples in 0.1 M borate buffer, pH 9.3. Denaturing Size-exclusion Chromatography--For the denaturing SEC, cells were lysed in PBS containing inhibitors as above and supplemented with 4 SDS, 25 mM TCEP, and 50 mM N-ethylmaleimide. The col