Transfer of Proteins across Membranes

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Abstract
Canine pancreas was fractionated into free ribosomes and rough microsomes. Detached ribosomes were prepared by treatment of rough microsomes with detergent. Poly(A)‐containing mRNA was extracted from rough microsomes. The biosynthesis of canine pancreatic secretory proteins was studied by comparing proteins synthesized in vitro by translation of mRNA or by completion of nascent chains present in free ribosomes, rough microsomes, and detached ribosomes with proteins synthesized in tissue slices using polyacrylamide gel electrophoresis in sodium dodecyl sulfate and subsequent autoradiography.The banding pattern of authentic secreted proteins synthesized in tissue slices was largely congruent with that obtained from the translation products of rough microsomes indicating that the bulk of the mRNA engaged with rough microsomes codes for secretory proteins.The banding pattern of translation products from mRNA in the absence of microsomal membranes was not congruent with that of authentic secretory proteins. Primary translation products for trypsinogen and the other serine protease zymogens using mRNA appeared to be larger in molecular weight than authentic proteins by 1000–2000 and are thus designated ‘presecretory’ proteins.The banding pattern from the translation products of free ribosomes, which are essentially devoid of membranes, was similar to that of ‘presecretory’ proteins.Translation of mRNA in the presence of microsomal membranes yielded a banding pattern for serine protease zymogens congruent with that of the translation products of rough microsomes, and these products were resistant to posttranslational proteolysis, indicating that segregation and processing of these polypeptide chains had taken place during translation in vitro.

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