Surface views of nuclear pores in isolated rat liver nuclei as revealed by fracture-flip/Triton-X.

We report here the use of fracture-flip (Anderson Forsman, C., P. Pinto da Silva, J. Cell Sci. 90, 531-541 (1988] and fracture-flip/Triton X-100 to examine the cytoplasmic surface of the outer nuclear membrane, and the perinuclear (exoplasmic) surface of the inner nuclear membrane of rat liver nuclei. After freeze-fracture, carbon stabilization and thawing, unfixed nuclei were treated with Triton X-100 to dissolve unfractured portions of the outer nuclear membrane. Flipping of the casts with their carbon-stabilized membrane halves (cytoplasmic half of the outer nuclear membrane; exoplasmic half of the inner nuclear membrane) exposed the cytoplasmic surface of the outer nuclear membrane and the surface of the inner nuclear membrane facing the perinuclear space. After drying, these membrane surfaces were visualized by Pt/C shadowing. We show that the cytoplasmic surface of outer nuclear membrane is covered by numerous large, oblong "kernel-like" globular particles (30-35 nm length, 25-30 nm width), often aligned into curved chains or grouped into clusters. Because of their size and characteristic arrays, we interpret these particles as ribosomes. Over the cytoplasmic surfaces of the outer nuclear membrane the nuclear pore complexes (outer diameter: 120-130 nm) appear as relatively inconspicuous annular structures, only slightly raised against the surrounding areas of membrane surface. Observation of the surface of the inner nuclear membrane facing the perinuclear space reveals prominent, sharply raised, "doughnut-like" nuclear pore complexes (outer diameter: 120-130 nm; inner diameter: 40 nm). Our results indicate the absence of large, protruding, ribosome-like structures lining the nuclear pore at its cytoplasmic surface, as proposed before.(ABSTRACT TRUNCATED AT 250 WORDS)