Đang chuẩn bị liên kết để tải về tài liệu:
Viral Hepatitis - part 3

Tuy nhiên, như các peptide tín hiệu của protein HBe không phải là đặc biệt mạnh mẽ, một phần quan trọng của protein HBe vẫn cytoplasmic.127 lượng protein HBe tổng hợp liên quan đến các protein HBV khác có thể quy định mức độ mRNA có chứa codon khởi đầu của một không bị gián đoạn gen HBe. | 168 Chapter 10 don. A purine at the 4 and or -3 position is usually found in initiation codons but not in internal codons for methionine.251 Additionally a stem-loop structure on the RNA about 30 bases downstream of the start codon can enhance the start of protein synthesis.252 Synthesis stops at one of the three possible stop codons UAA UAG or UGA which occurs first in this particular ORF. Thus the position of the 5 ends of the mRNAs in the HBV genome determines which gene product is expressed. Exceptions to this rule are internal ribosome entry sites IRES leaky scanning for start codons usage of atypical start codons read-through of stop codons frame shifting and reinitiation. Some of these exceptions may apply to the translation of certain HBV proteins. An important early step in protein biosynthesis is the distribution of the protein synthetic complex between the cytosol and the ER. Nascent protein chains that contain a hydrophobic a-helical stretch of 16-20 amino acids bind to a signal recognition particle that attaches the ribosome to a pore-like structure at the ER. The growing peptide is transported through this pore to the lumen of the ER. The particular structure and number of such hydrophobic signal sequences determine the intracellular distribution membrane topology or secretion of the protein. Proteins without signal sequence are synthesized by free ribosomes and released after completion to the cytosol. Enveloped viruses with subgenomic mRNAs such as HBV normally use free ribosomes for synthesis of their nucleocapsid proteins and ER-bound ribosomes for synthesis of the enveloped protein Fig. 10.5 . HBe protein The HBe protein is translated from the longest mRNA. As mentioned above this RNA differs from the preg-enomic mRNA in that it comprises an additional start codon at its 5 end. The translation initiation at this precore start suppresses the use of downstream-located AUGs.253 This includes the core start codon that is in frame with the precore .