DIFFERENTIAL EXPRESSION OF FIBROMODULIN MESSENGER-RNA ASSOCIATED WITHTENDON FIBRIL GROWTH - ISOLATION AND CHARACTERIZATION OF A CHICKEN FIBROMODULIN CDNA

A 450 bp cDNA fragment similar to that encoding bovine fibromodulin wa s isolated using a screening procedure to isolate genes differentially expressed between the pre- and post-growth phases of fibril growth in the developing chicken embryo metatarsal tendon. Using this fragment, a 2.4 kb cDNA clone for chicken fibromodulin was isolated from a lamb da ZAP library, and the 5' rapid amplification of cDNA ends technique was employed to clone the 5' end of the fibromodulin cDNA. The full-le ngth cDNA contained an open reading frame coding for a 380-amino-acid protein. There was similar to 80% similarity with human, rat and bovin e fibromodulins, which confirmed its identity as fibromodulin. Structu ral features of the deduced sequence include an 18-amino-acid signal p eptide, cysteine residues in conserved positions in the N- and C-termi nal regions, and a central leucine-rich domain containing eleven repea ts of the sequence LXXLXLXXNXL/I. Features unique to chicken fibromodu lin include an additional glycosylation site as well as a decreased nu mber of tyrosine residues that could be sulphated, and therefore poten tial changes in the charge of the molecule. In addition, there was lit tle similarity among the untranslated regions. When compared with chic ken decorin and lumican, fibromodulin showed greater similarity to the other keratan sulphate-containing proteoglycan, lumican. Northern blo t analysis revealed a 6-8-fold increase in the fibromodulin mRNA level from day 14 to day 19 of development. In the chicken tendon, collagen fibril growth is a process characterized by a precipitous increase in length during a short developmental period. The necessary changes wou ld require the expression of different genes regulating fibril formati on and growth, and interactions between fibromodulin and collagen fibr ils may participate in the regulation of collagen fibril growth and ma trix assembly.