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Coculture Analysis of Extracellular Protein Interactions Affecting Insulin Secretion by Pancreatic Beta Cells
Published on: June 15, 2013
Proinsulin intermolecular interactions during secretory trafficking in pancreatic β cells
Leena Haataja1, Erik Snapp, Jordan Wright
1Division of Metabolism, Endocrinology, and Diabetes, University of Michigan Medical School, Ann Arbor, Michigan 48105, USA.
The Journal of Biological Chemistry
|December 11, 2012
Summary
Human proinsulin normally accumulates in the Golgi, not the ER, indicating a slow step there. Misfolding shifts this rate-limiting step back to the ER.
Area of Science:
- Cell Biology
- Endocrinology
- Protein Trafficking
Background:
- Endoplasmic reticulum (ER) exit is typically rate-limiting for secretory proteins due to folding.
- Pancreatic beta cells process proinsulin into insulin and C-peptide.
Purpose of the Study:
- To investigate the steady-state localization and trafficking of human proinsulin in pancreatic beta cells.
- To identify the rate-limiting step in proinsulin secretion and its regulation.
Main Methods:
- Utilized a human proinsulin construct with superfolder green fluorescent C-peptide (hPro-CpepSfGFP) in pancreatic beta cells.
- Analyzed proinsulin localization using microscopy and assessed protein interactions via co-precipitation and FRET.
- Manipulated cellular conditions including fasting/refeeding and zinc deficiency.
Main Results:
- Proinsulin predominantly accumulated in the Golgi, suggesting a post-ER rate-limiting step.
- Golgi localization was dynamic, affected by fasting/refeeding and zinc levels.
- ER-entrapped mutant proinsulin shifted wild-type proinsulin localization back to the ER.
- Proinsulin dimerization occurs in the ER, but accumulation occurs later in the Golgi.
Conclusions:
- Proinsulin secretion involves a slow step in the Golgi, potentially related to hexamerization or granule formation.
- Proinsulin misfolding in beta cells can alter the rate-limiting step, shifting it to the ER.
- These findings challenge the classical view of ER exit as the sole rate-limiting step for proinsulin trafficking.
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