Topochemistry of hydrolytic enzymes in relation to Golgi apparatus in transplanted spontaneous mammary cancer of C3H mice.
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The sub-cellular localization of a native protein constitutes one coarse-grained aspect of its function. Transport between compartments is often regulated through short sequence motifs. Here, we analyzed experimentally characterized endoplasmic reticulum (ER)/ Golgi retrieval motifs and investigated the accuracy of homology-transfer. Only the C-terminal ER retrieval motifs KDEL, HDEL and AIAKE were sufficiently specific. However, even unspecific motifs may help, provided we know the probability for localization given the motif. We provided such estimates. We also rigorously estimated the accuracy and coverage for inferring ER and Golgi localization through homology-transfer by sequence similarity. In entire proteomes, we could thereby annotate 3304 ER (3182 membrane) and 1853 Golgi (759 membrane) proteins. We identified another putative 5157 globular and 3941 membrane ER or Golgi proteins. Each experimental annotation yielded, on average, one to three high-accuracy and five to six low-accuracy homology-transfers in the six proteomes. These numbers will increase with each new experimental annotation.
It has previously been shown that the steroidogenic action of adrenocorticotropic hormone (ACTH) is accompanied by characteristic alterations in cell ultrastructure. These include hypertrophy of the Golgi complex associated with increased vesicle formation and striking elevations of acid phosphatase activity in the Golgi complex and lysosomes. To investigate a possible relationship of these phenomena to steroidogenic function in monolayer cultures of murine adrenal tumor cells, monensin, a carboxylic ionophore which disrupts the ordered structure and transport function of the Golgi complex, was used. Monensin, at a concentration of 1.2 microM, causes massive vacuolization and hypertrophy of the Golgi complex. No effect on mitochondrial structure was seen. Monensin, 0.6-1.2 microM, inhibits both ACTH-stimulated and basal steroidogenesis by approximately 50% in incubations of 4 h or less. Dibutyryl-cAMP-stimulated steroidogenesis was inhibited to a similar degree. Incubations were carried out in serum-free media to eliminate possible effects due to exogenous cholesterol transport into the cell. There were no direct inhibitory effects of monensin on cholesterol side-chain cleavage (SCC) activity in isolated mitochondria. In contrast, mitochondria isolated from cells previously treated with monensin had a reduced capacity for this activity. These experiments suggest that monensin inhibits transport of cholesterol from the Golgi complex to the mitochondrial site of steroidogenesis action or interferes with the transport of key mitochondrial proteins synthesized on cytoplasmic ribosomes.
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