Crystal Structure of the LG1-3 Region of the Laminin α2 Chain

Abstract

Laminins are large heterotrimeric glycoproteins with many essential functions in basement membrane assembly and function. Cell adhesion to laminins is mediated by a tandem of five laminin G-like (LG) domains at the C terminus of the α chain. Integrin binding requires an intact LG1-3 region, as well as contributions from the coiled coil formed by the α, β, and γ chains. We have determined the crystal structure at 2.8-Å resolution of the LG1-3 region of the laminin α2 chain (α2LG1-3). The three LG domains adopt typical β-sandwich folds, with canonical calcium binding sites in LG1 and LG2. LG2 and LG3 interact through a substantial interface, but LG1 is completely dissociated from the LG2-3 pair. We suggest that the missing γ chain tail may be required to stabilize the interaction between LG1 and LG2-3 in the biologically active conformation. A global analysis of N-linked glycosylation sites shows that the β-sandwich faces of LG1 are free of carbohydrate modifications in all five laminin α chains, suggesting that these surfaces may harbor the integrin binding site. The α2LG1-3 structure provides the first atomic view of the integrin binding region of laminins.

Cross-linking reveals laminin coiled-coil architecture

“…Most of the cross-linked peptides were in the coiled coil, but some were within or between globular domains and in the EGF-like repeats. Crystal structures of laminin fragments (14,15,17,18) were used to assess the quality of the cross-linking results (Fig. 2).…”

“…Notably, one function of the coiled-coil terminus may be to stabilize the quaternary structural arrangement of the LG domains and support their adhesion functions. The structures of all five laminin LG domains were previously determined in two sets (14,15). In the structure of LG1 through…”

“…LG3, LG1 makes a domain swapping interaction to the LG2-LG3 pair from a different molecule (15), as the tight cloverleaf association of contiguous LG1, LG2, and LG3 domains suggested by electron microscopy (43) was lost. We observed two cross-links between LG1 and LG2-LG3: a zero-length link between LG1 and LG2, and an SDH link between LG1 and the LG2-LG3 junction.…”

“…Due to the centrality of laminin in cell-ECM interactions, efforts have been made to analyze the functional regions of the trimer, to determine which α, β, and γ paralogs associate into physiological heterotrimers and to understand how trimers self-assemble into higher-order networks. Laminin fragments have been generated to assess their binding properties (11,12) and as targets for structure determination by X-ray crystallography (13)(14)(15)(16)(17)(18)(19)(20).…”

The laminin family

“…The LG1 to LG3 trio is separated by a short stretch of amino acids from the LG4-LG5 pair. The crystal structure has been determined for recombinant LG1 to LG3 trio of the laminin a2 chain 24 and LG4-LG5 pair of the laminin a1 and a2 chains. 25,26 It indicates that LG domains adopt β-sandwich folds, with canonical calcium binding sites.…”