Prefracture and cold-fracture images of yeast plasma membranes.

Abstract

Fracture-temperature related differences in the ultrastructure of plasmalemma P [protoplasmic] faces of freeze-fractured baker''s yeast (Saccharomyces cerevisiae) were observed in high-resolution replicas. Prepared in freeze-etch EM systems and pumped to 2 .times. 10-7 torr, the specimens were protected from contamination by use of liquid nitrogen-cooled shrouds. Two major P-face images were observed regardless of the source of the yeast, the age of the culture, the growth temperature, the physiological condition or the suspending medium used: a cold-fracture image with many strands closely associated with tubelike particles (essentially the same image as those previously published for yeast freeze-fractured at 77.degree. K), and a prefracture image characterized by the presence of more distinct tubelike particles with few or no associated strands (for aging cultures, the image recently referred to as paracrystalline arrays of craterlike particles). Both types of P-face image can be found in separate areas of single replicas, and occasionally even within a single plasma membrane. While portions of replicas known to be fractured at any temperature colder than 218.degree. K reveal only the cold-fracture image, prefracture images are found in cells intentionally fractured at 243.degree. K and in cracks or fissures which develop during the freezing of other specimens. The prefracture image results from the fracturing of specimens at some temperature above 230.degree. K, not from fracturing specimens at some temperature between 173.degree. and 77.degree. K, and not from the use of starved yeast cells.