The usher protein has a molecular weight ranging from 86 to 100 kDa and is composed of a membrane-spanning 24-stranded beta barrel domain, reminiscent of porins, and of four periplasmic soluble domains : an N-terminal one of about 120 residues ( NTD ), located as a soluble insertion within the beta barrel region of the sequence ( plug domain ) and two IG-like domains ( each about 80 residues long ) at the C-terminus ( CTD1 and CTD2 ).
