Were placed in centrifugation tubes containing Microcon YM-10 filters (10-kDa cutoff; Nanosep?Centrifugal Devices with OmegaTM Membrane, 10 k). For further details, please see Ref. 32. Samples were centrifuged (14,000 rcf for 15 min) in urea buffer #1 (8 M urea, 1 mM dithiothreitol, 25 mM HEPES, pH 7.6). The material retained on the filter was then alkylated with urea buffer #2 (4 M urea, 55 mM io