THE FILAMENTOUS PHAGE PIV MULTIMER VISUALIZED BY SCANNING-TRANSMISSION ELECTRON-MICROSCOPY

A family of homomultimeric outer-membrane proteins termed secretins me diates the secretion of large macromolecules such as enzymes and filam entous bacteriophages across bacterial outer membranes to the extracel lular milieu. The secretin encoded by done filamentous phage fl was pu rified. Mass determination of individual molecules by scanning transmi ssion electron microscopy revealed two forms, a unit multimer composed of about 14 subunits and a multimer dimer. The secretin is roughly cy lindrical and has an internal diameter of about 80 angstroms, which is large enough to accommodate filamentous phage (diameter of 65 angstro ms).