HUVEC and KG1a cDNA library-derived CD109 cDNAs and predicted protein.

(A) Aligned restriction maps of the largest HUVEC (H6, H7)– and KG1a (K1)–derived CD109-specific cDNA fragments and the positions of the corresponding coding and untranslated regions are shown. K1 and deduced K1-H7 cDNAs contain identical ORFs but divergent 3′UTRs. White box, UTR; gray box, coding sequence; Aⁿ, poly(A) tail; Sm,Sma I; E, EcoRI; Sa, SacI; B, BamHI; H,HindIII; Xb, XbaI; P, PstI. (B) CD109 is a novel GPI-linked member of the α2M family of thioester-containing proteins. The translated K1 sequence (Figure 1) predicts a 1445-aa protein of about 162 kd bearing a cleavable 21-aa N-terminal leader peptide and a C-terminal consensus GPI anchor cleavage–addition signal, with cleavage predicted to occur after aa 1420. CD109 shares the overall domain structure of the α2M family, containing a thioester signature sequence (aa 918-924) approximately two thirds of the way along the molecule, a thioester reactivity-defining hexapeptide (aa 1030-1035) ending in VIH about 100 aa further downstream and a putative bait region (approximately aa 651-683) lying roughly in the middle of the protein.