lysosomal proteins) or secretion.

1958 Radioactive proteins are followed after their synthesis as they progress towards their secretory fate; this allows the definition of not only trafficking pathways but of the organelles that lie along that pathway.

The role of ribonucleoprotein in protein synthesis

5 The effects of CCK‐PZ and acetylcholine on pancreatic protein synthesis are controversial.

Dissociation of intracellular transport from protein synthesis

Mammalian cells export proteins, such as interleukin-1b, basic fibroblast growth factor, and thioredoxin, that do not have a classical signal sequence (31). In addition, drugs such as monensin and brefeldin A that normally block classical secretion are without effect on these proteins. Their secretion has been linked to the heat-shock response. Little is currently known about the molecular mechanisms that are involved.

Synthesis and turnover of membrane proteins

N2 - The rate of synthesis and the turnover of cytoplasmic membrane proteins were determined in the acinar cells of guinea pig pancreas with the aim of investigating the mechanisms by which the intracellular transport of secretion products occurs. These cells are highly specialized toward protein secretion. By means of in vitro pulse chase experiments and in vivo double labeling experiments, using radioactive L leucine as the tracer, it was found that the turnover of secretory proteins is much faster than that of all membranes involved in their transport (rough and smooth microsome and zymogen granule membranes). Sodium dodecyl sulfate polyacrylamide disk gel electrophoresis of membrane proteins revealed that in each of these membranes there is a marked heterogeneity of turnover; generally the high molecular weight polypeptides have a shorter half life than the low molecular weight polypeptides. These data indicate that the membranes participating in the intracellular transport of secretory proteins are not synthesized concomitantly with the latter. Rather, they are probably reutilized in several successive secretory cycles. The possible relevance of these findings to other secretory systems is discussed.

An ultrastructural and cytochemical study of the isoproterenol‐induced secretory cycle
Synthesis and transport of glycoproteins and proteoglycans in the apical and basolateral secretory pathways of epithelial MDCK cells.

The production of a secretory protein starts ..

When E. coli makes a protein that is destined for export, it is made as a preprotein containing a signal sequence (4). In most proteins, these signal sequences are a stretch of contiguous amino acids at the amino terminus of a protein, with a positively charged amino terminus, at least six sequential hydrophobic amino acids, and a cleavage site for the signal peptidase that removes the signal after translocation. The presence of the signal sequence in a newly synthesized protein causes a protein chaperone to bind to it, retarding its folding. There appear to be at least two major chaperone systems in E. coli. One involves the tetrameric cytosolic protein secB, which binds to newly synthesized proteins that contain a signal sequence. It binds throughout the length of the newly synthesized polypeptide chain, keeping it from adopting its native conformation (2). A different class of chaperones binds to the nascent chain as the secreted protein is being synthesized on the ribosome. An example of such a chaperone is a ribonucleoprotein, the signal recognition protein (SRP). In E. coli SRP consists of a 4.5 S RNA molecule and a single protein, Ffh, which is a GTPase; that is, it hydrolyzes guanosine triphosphate (5).

What is the pathway of secretory proteins? - Quora

Experiments have been carried out to determine whether intracellular transport of pancreatic secretory proteins is obligatorily coupled to protein synthesis or whether it is a separable process which can be independently regulated. To this intent, guinea pig pancreatic slices were pulse labeled with leucine-3H for 3 min and incubated post-pulse for 37 min in chase medium containing cycloheximide up to concentrations sufficient to inhibit protein synthesis by 98%. In controls, newly synthesized secretory proteins are transported over this interval to condensing vacuoles of the Golgi complex. Since the latter are recovered in the zymogen granule fraction upon cell fractionation, intracellular transport was assayed by measuring the amount of protein radioactivity found in the zymogen granule fraction after a (3 + 37) min incubation. The results indicated that at maximum inhibition of protein synthesis (5 x 10-4 cycloheximide), transport proceeded with an efficiency ~80% of control. Parallel radioautographic studies on intact slices confirmed these data and further indicated that all the steps of intracellular transport, including discharge to the acinar lumen, were independent of protein synthesis. We conclude that: (1) transport and protein synthesis are separable processes; (2) intracellular transport is not the result of a continuous delivery of secretory proteins from attached polysomes to the cisternae of the rough endoplasmic reticulum; and (3) transport is not dependent on the synthesis of "specific" nonsecretory proteins within the time limits tested.

Biosynthetic protein transport in the secretory pathway W.B

Bacteria may push protein across the translocation channel, rather than pull it, because the source of energy, ATP, is exclusively cytoplasmic. Once in the periplasmic space, secreted proteins form disulfide bonds and fold into proteinase-resistant conformations. Such folding is necessary before the proteins are transported through holes, poorly described, in the outer membrane.