Molecular cloning and genomic organization of chicken syndecan-4

Abstract

We have cloned and determined the genomic organization of the core protein of the chicken transmembrane proteoglycan, syndecan-4. Identification of the initial cDNA was accomplished using polyclonal antibodies directed against the cytoplasmic domain of murine syndecan-1 core protein. The cDNA for chicken syndecan-4 encodes a putative core protein of 197 amino acids which consists of a 19-amino acid signal peptide, a 125-amino acid ectodomain, a 25-amino acid transmembrane domain, and a 28-amino acid cytoplasmic domain. The predicted molecular mass of the mature core protein is 19,639 daltons. The ectodomain of chicken syndecan-4 core protein contains three potential sites for glycosaminoglycan attachment, two sites for N-glycosylation, and lacks a dibasic protease cleavage site proximal to the membrane-spanning region found in other syndecan family members. Comparison of the complete amino acid sequence with human syndecan-4 (amphlican (David, G., van der Schueren, B., Marynen, P., Cassiman, J. J., and van den Berghe, H. (1992) J. Cell Biol. 118, 961-969)) and rat syndecan-4 (ryudocan (Kojima, T., Shworak, N. W., and Rosenberg, R. D. (1992) J. Biol. Chem. 267, 4870-4877)) indicates an overall identity of 58 and 56%, respectively, with a 91 and 92% identity in the highly conserved transmembrane and cytoplasmic domains. The core protein of chicken syndecan-4 synthesized by chicken cells is modified with heparan sulfate side chains yielding a proteoglycan with a molecular mass of > 200 kDa in LMH cells (immortalized male leghorn LM strain hepatocytes) and primary skin fibroblasts. Syndecan-4 isolated from chondrocyte cultures runs as a diffuse band between 100 and 200 kDa. Northern analysis of chicken syndecan-4 indicates three messages with distinct sizes of 0.9, 1.3, and 2.9 kb and a wide mRNA tissue distribution. The chicken syndecan-4 gene is divided into 5 exons encoding distinct regions which contain the signal peptide, the glycosaminoglycan attachment sites, a small spacer of unknown function, the glycosylation sites and the transmembrane and cytoplasmic domains.