Biological Activities of Homologous Loop Regions in the Laminin α Chain G Domains*

Abstract

Laminin α chains (α₁–α₅ chains) have diverse chain-specific biological functions. The LG4 modules of laminin α chains consist of a 14-stranded β-sheet (A-N) sandwich structure. Several biologically active sequences have been identified in the connecting loop regions. Here, we evaluated the biological activities of the loop regions of the E and F strands in the LG4 modules using five homologous peptides from each of the mouse α chains (EF-1: DYATLQLQEGRLHFMFDLG, α₁ chain 2747–2765; EF-2: DFGTVQLRNGFPFFSYDLG, α₂ chain 2808–2826; EF-3: RDSFVALYLSEGHVIFALG, α₃ chain 2266–2284; EF-4: DFMTLFLAHGRLVFMFNVG, α₄ chain 1511–1529; EF-5: SPSLVLFLNHGHFVAQTEGP, α₅ chain 3304–3323). These homologous peptides showed chain-specific cell attachment and neurite outgrowth activities. Well organized actin stress fibers and focal contacts with vinculin accumulation were observed in fibroblasts attached on EF-1, whereas fibroblasts on EF-2 and EF-4 showed filopodia with ruffling. Fibroblast attachment to EF-2 and EF-4 was mediated by syndecan-2. In contrast, EF-1 promoted α₂β₁ integrin-mediated fibroblast attachment and inhibited fibroblast attachment to a recombinant laminin α₁ chain LG4-5. The receptors for EF-3 and EF-5 are unknown. Further, when the active core sequence of EF-1 was cyclized, utilizing two additional cysteine residues at both the N and C termini through a disulfide bridge, the cyclic peptide significantly enhanced integrin-mediated cell attachment. These results indicate that integrin-mediated cell attachment to the EF-1 sequence is conformation-dependent and that the loop structure is important for the activity. The homologous peptides, which promote either integrin- or syndecan-mediated cell attachment, may be useful for understanding the cell type- and chain-specific biological activities of the laminins.