MOLECULAR-CLONING AND GENOMIC ORGANIZATION OF CHICKEN SYNDECAN-4

We have cloned and determined the genomic organization of the core pro tein of the chicken transmembrane proteoglycan, syndecan-4. Identifica tion of the initial cDNA was accomplished using polyclonal antibodies directed against the cytoplasmic domain of murine syndecan-1 core prot ein. The cDNA for chicken syndecan-4 encodes a putative core protein o f 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 synd ecan-4 core protein contains three potential sites for glycosaminoglyc an attachment, two sites for N-glycosylation, and lacks a dibasic prot ease cleavage site proximal to the membrane-spanning region found in o ther syndecan family members. Comparison of the complete amino acid se quence with human syndecan-4 (amphlican (David, G., van der Schueren, B., Marynen, P., Cassiman, J. J., and van den Berghe, H. (1992) J. Cel l Biol. 118, 961-969)) and rat syndecan-4 (ryudocan (Kojima, T., Shwor ak, 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 chicke n cells is modified with heparan sulfate side chains yielding a proteo glycan with a molecular mass of >200 kDa in LMH cells (immortalized ma le leghorn LM strain hepatocytes) and primary skin fibroblasts. Syndec an-4 isolated from chondrocyte cultures runs as a diffuse band between 100 and 200 kDa. Northern analysis of chicken syndecan-4 indicates th ree messages with distinct sizes of 0.9, 1.3, and 2.9 kb and a wide mR NA 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 domain s.