AIDA is a 1286-residue outer membrane protein belongs to the family of autotransporters, which in Gram-negative bacteria is used to display proteins on the cell surface, either as a monomer or a multimeric pore. The membrane-bound C-terminal domain (AIDAb, residues 951-1286) is believed to form a 14-strand b-barrel. AIDA is purified in urea from inclusion bodies. Upon transfer to lipids or detergents, it forms a compact and cooperatively folded state C which nevertheless is protease-sensitive. C may thus represent a late folding intermediate. Using double-jump assays to monitor formation of C, it is shown that folding of C takes minutes to hours, competes with aggregation and requires at least partial insertion into a membrane environment. We have recently been able to fold AIDA to a protease-resistant form, and are currently characterizing its folding properties. We have purified a number of periplasmic chaperones and demonstrated that SurA appears to accelerate AIDA folding. AFM and FTIR analysis of AIDA in different lipid environments is being carried out. The biophysical properties of the outer membrane protein flocculating agent Ag43 have been investigated. (Jesper E. Mogensen, Tony Ebdrup, Lise Nesgaard, Jeppe Jensen, Daniel Otzen).
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