Hemicentin assembly in the extracellular matrix is mediated by distinct structural modules

Hemicentins are conserved extracellular matrix proteins characterized by a single von Willebrand A (VWA) domain at the amino terminus, a long stretch (>40) of tandem immunoglobulin domains, multiple tandem epidermal growth factors (EGFs), and a single fibulin-like carboxyl-terminal module. In Caenorhabditis elegans, hemicentin is secreted from muscle and gonadal leader cells and assembles at multiple locations into discrete tracks that constrict broad regions of cell contact into adhesive and flexible line-shaped junctions. To determine hemicentin domains critical for function and assembly, we have expressed fragments of hemicentin as GFP tagged fusion proteins in C. elegans. We find that a hemicentin fragment containing the VWA domain can target to multiple assembly sites when expressed under the control of either endogenous hemicentin regulatory sequences or the muscle-specific unc-54 promoter. A hemicentin fragment containing the EGF and fibulin-like carboxyl-terminal modules can co-assemble with existing hemicentin polymers in wild-type animals but has no detectable function in the absence of endogenous hemicentin. The data suggest that the VWA domain is a cell binding domain whose function is to target hemicentin to sites of assembly and the EGF/fibulin-like carboxyl-terminal modules constitute an assembly domain that mediates direct interactions between hemicentin monomers during the hemicentin assembly process.