Figure 2.

(a) Separation of aggregates and monomers from the pentameric CTB^GPGPMPR protein by gel filtration on a Superdex 200 column. Assembly status was estimated from parallel resolution of molecular-mass standards (not shown). Inset, tryptophan fluorescence emission spectra of pentameric CTB^GPGPMPR in pooled gel-filtration fractions corresponding to the major peak in (a). 1 (green), pentamers; 2 (blue), concentrated (Centricon 100) pentamers. Excitation was at 280 nm. (b) Proteins in the unconcentrated metal-affinity chromatography (MAC) eluate and in the size-exclusion chromatography (SEC) fraction corresponding to the main peak of the chromatogram in (a) were resolved by SDS–PAGE under nondenaturing (ND; no DTT and no boiling) and denaturing (D) conditions. Molecular-weight standards indicate that CTB^AAAAMPR is organized into SDS-stable pentamers. The compact pentamers have a slightly higher electrophoretic mobility than expected based on their mass alone.