SURFACE-ANALYSIS OF THE PHOTOSYSTEM-I COMPLEX BY ELECTRON AND ATOMIC-FORCE MICROSCOPY

Two-dimensional (2D) crystals of the photosystem I(PSI) reaction cente r from Synechococcus sp. OD24 were analyzed by electron and atomic for ce microscopy. Surface relief reconstructions from electron micrograph s of freeze-dried unidirectionally shadowed samples and topographs rec orded with the atomic force microscope (AFM) provided a precise defini tion of the lumenal and stromal PSI surfaces. The lumenal surface was composed of four protrusions that surrounded an indentation. One of th e protrusions, the PsaF subunit, was often missing. Removal of the ext rinsic proteins with the AFM stylus exposed the stromal side of the PS I core, whose surface structure could then be imaged at a resolution b etter than 1.4 nm. This interfacial surface between core and extrinsic subunits, had a pseudo-2-fold symmetry and protrusions that correlate d with the surface helices e and e' or were at the sites of putative c c-helix-connecting loops estimated from the 4 Angstrom map of the comp lex. The molecular dissection achieved with the AFM, opens new possibi lities to unveil the interfaces between subunits of supramolecular ass emblies. (C) 1998 Academic Press.