New method for localizing proteins in periodic structures: Fab fragment labeling combined with image processing of...

Fab fragments prepared from antisera directed against purified bacteriophage T4 structural proteins and head-related structures were used to label proteins on the surface of T-even giant phage capsids. Optically filtered electron micrographs of the Fab-labeled capsids reveal both the location of specific proteins within the capsomeres and differing conformational states of the protein subunits. We describe parameters affecting the utility of this technique for the study of molecular organization and protein conformation in periodic biological structures.