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 inhibi ting protein synthesis and/or by subjecting thermosensitive secretory mutan ts to double temperature shifts. Five membrane-bounded structures disappear ed or reappeared in an orderly manner at approximately the rate of secretor y protein flow. 1) The first detectable post-ER intermediates were very sho rt-lived clusters of small vesicles that appeared next to the endoplasmic r eticulum (ER). 2) Their constituent small vesicles were rapidly bridged by membrane tubules in a SEC18-dependent manner, giving short-lived tubular cl usters 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 struct ure successively reappeared, seemingly by transformation of the previous on e. When no secretory cargo was to be transported, these structures were not renewed. They disappeared more than five times faster than some Golgi enzy mes such as Och1p, implying that the latter are recycled and perhaps partia lly retained. Retention could arise from intra-compartmental flow of cargo/ carrier, hinted at by the varying calibers within a single nodular network.