Morphogenesis and dynamics of the yeast Golgi apparatus

A kinetic and morphometric study was conducted with the electron microscope to clarify the biogenesis and structural diversity of the Golgi apparatus in the yeast Saccharomyces cerevisiae. Secretion was synchronized by inhibiting protein synthesis and/or by subjecting thermosensitive secretory mutants to double temperature shifts. Five membrane-bounded structures disappeared or reappeared in an orderly manner at approximately the rate of secretory protein flow. 1) The first detectable post-ER intermediates were very short-lived clusters of small vesicles that appeared next to the endoplasmic reticulum (ER). 2) Their constituent small vesicles were rapidly bridged by membrane tubules in a SEC18-dependent manner, giving short-lived tubular clusters of small vesicles, analogous to mammalian vesicular-tubular clusters. 3) Fine and 4) large nodular networks (coated with the Golgi protein Sec7), and 5) secretory granules. Upon relieving a secretory block, each structure successively reappeared, seemingly by transformation of the previous one. When no secretory cargo was to be transported, these structures were not renewed. They disappeared more than five times faster than some Golgi enzymes such as Och1p, implying that the latter are recycled and perhaps partially retained. Retention could arise from intra-compartmental flow of cargo/carrier, hinted at by the varying calibers within a single nodular network.