Disparate Effects of p24α and p24δ on Secretory Protein Transport and Processing

Jeroen R P M Strating,Gerard J M Martens,Theo G M Hafmans,Gerrit Bouw

doi:10.1371/journal.pone.0000704

Abstract

BackgroundThe p24 family is thought to be somehow involved in endoplasmic reticulum (ER)-to-Golgi protein transport. A subset of the p24 proteins (p24α3, -β1, -γ3 and -δ2) is upregulated when Xenopus laevis intermediate pituitary melanotrope cells are physiologically activated to produce vast amounts of their major secretory cargo, the prohormone proopiomelanocortin (POMC).Methodology/Principal FindingsHere we find that transgene expression of p24α3 or p24δ2 specifically in the Xenopus melanotrope cells in both cases causes an effective displacement of the endogenous p24 proteins, resulting in severely distorted p24 systems and disparate melanotrope cell phenotypes. Transgene expression of p24α3 greatly reduces POMC transport and leads to accumulation of the prohormone in large, ER-localized electron-dense structures, whereas p24δ2-transgenesis does not influence the overall ultrastructure of the cells nor POMC transport and cleavage, but affects the Golgi-based processes of POMC glycomaturation and sulfation.Conclusions/SignificanceTransgenic expression of two distinct p24 family members has disparate effects on secretory pathway functioning, illustrating the specificity and non-redundancy of our transgenic approach. We conclude that members of the p24 family furnish subcompartments of the secretory pathway with specific sets of machinery cargo to provide the proper microenvironments for efficient and correct secretory protein transport and processing.

Highlights

The secretory pathway consists of a number of distinct membranebounded subcompartments that have specialized functions in the process of protein biosynthesis [1]
F1 offspring was produced by in vitro fertilization of eggs harvested from wild-type females with sperm isolated from the testes of individual transgenic males or by in vitro fertilization of eggs harvested from individual transgenic females with sperm isolated from the testes of wild-type males
The type-I transmembrane p24 proteins are abundantly present in endoplasmic reticulum (ER)- and Golgi-derived transport vesicles, and are thought to play an important role in some aspect of cargoselective transport through the early secretory pathway

Summary

Introduction

The secretory pathway consists of a number of distinct membranebounded subcompartments that have specialized functions in the process of protein biosynthesis [1]. The ,24K p24 proteins constitute a family that can be subdivided into four subfamilies (p24a, b, c and d) [4], with each family member displaying multiple domains conserved from yeast to mammals (reviewed in [2]). In this report, ‘‘secretory cargo’’ refers to the biologically active transmembrane and soluble proteins that are transported to the plasma membrane or extracellular matrix as well as to the bioactive soluble proteins that are secreted into the extracellular space, whereas the machinery proteins or lipids that are supplied to subcompartments of the secretory pathway to provide the proper microenvironments for efficient and correct transport and processing of the secretory cargo are designated ‘‘machinery cargo’’. We conclude that members of the p24 family furnish subcompartments of the secretory pathway with specific sets of machinery cargo to provide the proper microenvironments for efficient and correct secretory protein transport and processing

Methods

Results

Conclusion