Secretion of Cartilage Oligomeric Matrix Protein Is Affected by the Signal Peptide

Paul Holden,Douglas R Keene,Gregory P Lunstrum,Hans Peter Bächinger,William A Horton

doi:10.1074/jbc.m411716200

Abstract

Cartilage oligomeric matrix protein (COMP) is a secreted glycoprotein found in the extracellular matrices of skeletal tissues. Mutations associated with two human skeletal dysplasias, pseudoachondroplasia and multiple epiphyseal dysplasia, disturb COMP secretion leading to intracellular accumulation of mutant COMP, especially in chondrocytes. Here we show that the manifestation of this secretory defect is dramatically influenced by the signal peptide that targets COMP for secretion. The comparison of wild type and mutant COMP secretion directed by the COMP or BM40 signal peptide in HEK-293 cells and rat chondrosarcoma cells revealed that the BM40 signal peptide substantially enhances secretion of mutant COMP that accumulates in endoplasmic reticulum-like structures when targeted by its own signal peptide. Additionally, we demonstrate that mutant COMP forms mixed pentamers with wild type COMP. Our findings suggest that the secretory defect in pseudoachondroplasia and multiple epiphyseal dysplasia is not specific for chondrocytes, nor does it require interaction of mutant COMP with other matrix proteins prior to transport from the cell. They also imply a previously unappreciated role for the signal peptide in the regulation of protein secretion beyond targeting to the endoplasmic reticulum.

Highlights

Cartilage oligomeric matrix protein (COMP)1 is a large pentameric glycoprotein found in the extracellular matrix of cartilage, tendon, ligament, synovium, and in the vasculature [1,2,3,4,5]
The Signal Peptide Affects Secretion of Wild Type and Mutant COMP—The PSACH-causing mutation del D469 and the multiple epiphyseal dysplasia (MED)-causing mutation del RG367–368 were introduced into the full-length mouse COMP cDNA by PCR-based mutagenesis
Because the hydrophobic region of the signal peptide has been shown to be important in the insertion of the signal peptide into the lipid bilayer of the rough endoplasmic reticulum (rER) and subsequent positioning of the cleavage site for access by signal peptidase, we compared the hydropathy profiles of the two signal peptides [43]

Summary

Introduction

Cartilage oligomeric matrix protein (COMP) is a large pentameric glycoprotein found in the extracellular matrix of cartilage, tendon, ligament, synovium, and in the vasculature [1,2,3,4,5] It is the fifth member of the thrombospondin family of proteins and, as such, has a modular domain structure comprised of an amino-terminal domain, four epidermal growth factor-like domains, eight-thrombospondin type III repeats, and a COOH-terminal globular domain [6]. Other matrix proteins known to interact with COMP including collagen types IX, XI, and XII, fibromodulin, and decorin have been co-localized with COMP to enlarged chondrocyte rER in patient tissues as well [25] These observations have led to speculation that extracellular deficiency of COMP and potentially of COMPinteracting proteins is responsible for the cartilage disease associated with COMP mutations. A common feature of signal peptides is a tripartite domain structure composed of a highly diverse NH2-terminal region, a central region of 7–15 hydrophobic residues, and a short stretch of 2–9 small polar residues wherein lies the motif for cleavage by signal peptidase [34]

Methods

Results

Conclusion