![]() ![]() ![]() The signal sequence binds to the signal recognition particle complex (SRP) and brings the ribosome, attached nascent protein, and mRNA to the ER membrane SRP-receptor 2. Also known as a leader sequence, the signal sequence allows nascent proteins to cross the endoplasmic reticulum (ER) membrane by traversing the translocon, a large multi-protein channel 1. Secretory proteins, including pancreatic digestive enzymes, contain a distinct n-terminal signal sequence which contains a 15- to 50-amino acid peptide that includes a hydrophobic core. The two subunits form a groove wherein new protein synthesis directed by messenger RNA takes place, aided by associated protein complexes to initiate, elongate, and terminate protein synthesis. The eukaryotic ribosome is composed of two subunits: a large 60S unit containing 28S, 5S, and 5.8S rRNA and approximately 49 proteins and a small subunit at 40S which includes 18S rRNA and about 33 proteins. ![]() The ribosome is the central element of the protein synthetic machinery. We present results primarily obtained using rodent acinar cells, though the limited data from human acinar cells suggest the functions are likely the same as in rodents. Here, we focus on acinar cell protein synthesis, trafficking, and processing in the pancreatic acinar cell necessary for its central role in producing digestive enzymes. After electron microscopy was developed, for example, cell biologists first visualized organelles and established their function by studying acinar cells ( Figure 1). The acinar cell has been a model system for foundational studies of protein synthesis and export. The exocrine pancreas has two major physiologic functions: it supplies the proenzymes and enzymes needed for digesting dietary lipids, carbohydrates, and proteins and secretes a bicarbonate-rich fluid that neutralizes acidic gastric secretions and thus provides the correct pH for intestinal digestion by pancreatic enzymes. ![]()
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