Ionic interactions in the coiled-coil domain of laminin determine the specificity of chain assembly

Laminins are a family of large (800 to 900 kDa) multidomain glycoproteins specifically found in basement membranes. They consist of one heavy A chain and two light chains B1 and B2, and several tissue-specific laminin isoforms exist. Chain assembly is mediated through the formation of a rod-like triple-stranded alpha-helical coiled-coil domain about 75 nm long. The interacting edges of the chains are mostly formed by hydrophobic residues in positions a and d of an (abcdefg)n heptad sequence repeat and by a distinct pattern of charged residues in positions e and g. Here, we have analyzed the sequences of known laminin chains in an effort to relate them to interaction potential. Initially, those sequences localized in the long arm were arranged in an optimum heptad-repeating scheme. The interacting edges between chains were then analyzed for interchain hydrophobic and ionic interactions. The short heptad blocks were allowed to shift axially with respect to each other to maximize the number of interactions. The number of hydrophobic interactions was very high and similar for all chain combinations, but especially so for homodimers. As these were not observed experimentally, it seems that hydrophobic interactions probably represent only a prerequisite for coiled-coil formation. The number of ionic interactions, however, directly resembles the interaction potential observed in in vitro experiments. In particular, the number of interchain ionic interactions is high for parallel heterodimer configurations of A and B chains, but low for homodimer arrangements. When the laminin isoform chains, rat s-laminin (B1s) and human merosin (Am), are included in the analysis, they show rather low numbers of mutual interactions but high ionic interaction potentials between them and distinct mouse laminin chains are predicted. For mouse laminin the analysis was extended to a full three-stranded coiled-coil structure. The highest number of interchain ionic interactions occurs for an anti-clockwise chain arrangement of A-->B1-->B2 when viewed from the N terminus. None of the laminin chains appears to be designed for the formation of homodimers, although such conformations are frequently found in other alpha-fibrous proteins.