STRUCTURE AND EXPRESSION OF FIBULIN-2, A NOVEL EXTRACELLULAR-MATRIX PROTEIN WITH MULTIPLE EGF-LIKE REPEATS AND CONSENSUS MOTIFS FOR CALCIUM-BINDING

A new protein, fibulin-2, was predicted from sequence analysis of cDNA clones obtained from a mouse fibroblast library. This protein consist s of a 1195-residue polypeptide preceded by a 26-residue signal peptid e. The COOH-terminal region of 787 amino acids contained three anaphyl atoxin-related segments (domain I), 11 EGF-like repeats (domain II), 1 0 of which had a consensus motif for calcium-binding, and a 115-residu e globular domain III. Except for two additional EGF-like repeats, thi s COOH-terminal region showed 43% sequence identity with the previousl y described fibulin-1 (BM-90). The NH2-terminal 408 residues, unique t o fibulin-2, showed no sequence homology to other known proteins and p resumably form two additional domains that differ in their cysteine co ntent. Recombinant fibulin-2 was produced and secreted by human cell c lones as a disulfide-bonded trimer. Rotary shadowing visualized the pr otein as three 40-45 nm long rods which are connected at one end in a globe-like structure. No significant immunological cross-reaction coul d be detected between fibulin-1 and fibulin-2. Production of the fibul in-2 was demonstrated by Northern blots and radioimmunoassay in fibrob lasts but not in several tumor cell lines. Together with the observati on that the serum level of fibulin-2 is 1,000-fold lower than that of fibulin-1, the data indicate that these two isoforms are not always co ordinately expressed. This is also suggested by Northern blots of tiss ue mRNAs and by immunofluorescence localizations using mouse tissues. The latter studies also demonstrated an extracellular localization for fibulin-2 in basement membranes and other connective tissue compartme nts.