SELF-ASSEMBLY OF LAMININ ISOFORMS

The alpha, beta, and gamma subunits of basement membrane laminins can combine into different heterotrimeric molecules with either three full short arms (e.g. laminins-1-4), or molecules containing one (laminins -6-9) or more (laminin-5) short arm truncations. Laminin-1 (alpha 1 be ta 1 gamma 1), self-assembles through a calcium-dependent thermal gela tion requiring binding interactions between N-terminal short arm domai ns, forming a meshwork polymer thought to contribute to basement membr ane architecture (Yurchenco, P. D., and Cheng, Y. S. (1993) J. Biol. C hem. 268, 17286-17299). However, it has been unclear whether other iso forms share this property, and if so, which ones. To begin to address this, we evaluated laminin-2 (alpha 2 beta 1 gamma 1), laminin-4 (alph a 2 beta 2 gamma 1), laminin-5 (alpha 3A beta 3 gamma 2), and laminin- 6 (alpha 3A beta 1 gamma 1). The first two isoforms were found to self -aggregate in a concentration- and temperature-dependent manner and a close self-assembly relationship among laminins-1, -2, and -4 were dem onstrated by: (a) polymerization of all three proteins was inhibited b y EDTA and laminin-1 short arm fragments, (b) polymerization of lamini n-1 was inhibited by fragments of laminins-2 and -4, (c) laminin-2 and , to a lesser degree, laminin-4, even well below their own critical co ncentration, co-aggregated with laminin-1, evidence for co-polymerizat ion. Laminin-5, on the other hand, neither polymerized nor co-polymeri zed with laminin-1. Laminin-6 failed to co-aggregate with laminin-l at all concentrations evaluated, evidence for a lack of a related self-a ssembly activity. The data support the hypothesis that all three short arms are required for self-assembly and suggest that the short arm do main structure of laminin isoforms affect their architecture-forming p roperties in basement membranes.