The Moraxella IgD-binding protein MID/Hag is an oligomeric autotransporter

The immunoglobulin D (IgD)-binding protein MID/Hag of the human respiratory pathogen Moraxella catarrhalis is an outer membrane protein of approximately 200 kDa belonging to the autotransporter family. MID also functions as an adhesin and hemagglutinin. In the present paper, the ultrastructure of MID was mapped. Using a series of Escherichia coli transformants, the last 210 aa of the C-terminal region were shown to translocate protein MID through the outer membrane suggesting that MID has a β-barrel structure comprising of 10 transmembrane β-sheets. Electron microscopy mapping with gold-labelled specific antibodies, and partial unravelling using guanidine hydrochloride showed that the rest of the MID protein forms an approximately 120 nm long, fibrillar structure in which the individual monomers fold back on themselves to expose a globular distal domain at their tips comprising both the IgD-binding (MID962–1200) and adhesive (MID764–913) regions. This positions their N-termini close to the C-terminal membrane spanning domains. Mass measurements by scanning transmission electron microscopy (STEM) verified that the MID molecule is an oligomer.