Fig. 5. sec21-3 mutant maintained for 5 minutes (a) and 20 minutes (b) at 37°C, blocked for 20 minutes at 37°C and returned for 5 (c,d) or 10 minutes (e,f,g) to permissive temperature (24°C). (a) After 5 minutes at 37°C, tubular fragments with nodular dilations (arrowhead) are interspersed within the cytoplasm. An ovoid mass made up of interconnected short ribbon-like ER elements (*) is continuous with the nuclear envelope. V, small vacuole; N, nucleus; SG, free secretion granules. Magnification x40,800. (b) After 20 minutes at 37°C, the secretion granules and tubular fragments with or without dilations are no longer seen. A mass of interconnected ribbon-like ER elements (*) forms a bridge between the nuclear envelope and the subplasmalemmal ER at the top-right. Two vacuole-like structures (V) are present next to the nucleus (N). Magnification x35,400. (c) As early as 5 minutes after shifting the temperature down to 24°C, fenestrations (white arrow) and tubular networks (arrow) with dilations (arrowheads) are seen at the periphery of an unperforated sheet of ER, which is itself continuous with the subplasmalemmal ER (ER). Magnification x42,000. (d) After 5 minutes at 24°C, a tubular network (arrow) with more intensely stained dilations (arrowhead) is observed. SG, a free secretion granule; CW, cell wall. Magnification x48,000. (e) At 10 minutes after shifting the cells back to 24°C, numerous secretion granules (SG) are interspersed within the cytoplasm and accumulate in the bud on the right. At the center left, an ER sheet seen in face view shows peripheral tubules (arrow) and dilations (arrowheads) with size and staining properties similar to those of secretion granules. Magnification x59,100. (f) After 10 minutes at 24°C, a broken tubular network, presumably a Golgi element (G), with dilations of various sizes and staining intensities is observed at the top-left. SG, free secretion granules; ER, an ER sheet with a fenestrated periphery. Magnification x35,000. (g) Small poorly contrasted vacuole-like structures (*) accumulate inside or in close proximity to a large intensely reactive vacuole (V). Magnification x23,300.
