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Processing in vitro of placental peptide hormones by smooth microsomes
Abstract:
Rough and smooth microsomes were prepared from ascites tumor cells, rat liver, and bovine adrenal cortex. Proteolytic removal of the signal peptide in pre-placental lactogen and asparagine-linked glycosylation of the alpha subunit of chorionic gonadotropin by these fractions were examined in mRNA-dependent lysates from ascites cells. Both processing steps were performed by smooth microsomes, which was unexpected because it has been presumed that only rough microsomes contain components for ribosomal binding. Thus smooth microsomes are apparently capable of interacting with polysomes bearing secretory nascent chains, and cleavage and asparagine-linked glycosylation activities are present in both rough and smooth endoplasmic reticulum.
Insights
Smooth microsomes, not just rough ones, process proteins like pre-placental lactogen and chorionic gonadotropin. This finding reveals that both rough and smooth endoplasmic reticulum have key protein processing activities.
Area of Science:
- Cell Biology
- Molecular Biology
- Biochemistry
Background:
- The endoplasmic reticulum (ER) is crucial for protein synthesis and modification.
- Rough microsomes are traditionally associated with protein synthesis due to ribosomal binding.
- Smooth microsomes were not typically considered involved in direct protein processing.
Purpose of the Study:
- To investigate the protein processing capabilities of smooth microsomes.
- To determine if smooth microsomes can perform signal peptide cleavage and glycosylation.
- To compare the activities of rough and smooth microsomes in protein maturation.
Main Methods:
- Preparation of rough and smooth microsomes from ascites tumor cells, rat liver, and bovine adrenal cortex.
- Analysis of signal peptide removal from pre-placental lactogen in mRNA-dependent lysates.
- Assay of asparagine-linked glycosylation for the alpha subunit of chorionic gonadotropin.
Main Results:
- Smooth microsomes effectively performed proteolytic removal of signal peptides from pre-placental lactogen.
- Smooth microsomes were also capable of asparagine-linked glycosylation of the alpha subunit of chorionic gonadotropin.
- Both rough and smooth microsomes exhibited these protein processing activities, challenging previous assumptions.
Conclusions:
- Smooth microsomes possess the machinery for signal peptide cleavage and N-linked glycosylation.
- The endoplasmic reticulum, encompassing both rough and smooth fractions, is fully equipped for these secretory protein processing steps.
- Smooth microsomes can interact with polysomes containing nascent secretory chains, expanding our understanding of ER function.