ROLE OF THE B1 SHORT ARM IN LAMININ SELF-ASSEMBLY

Laminin self-assembles into a basement membrane polymer through specif ic low-affinity interactions. Recently, it was shown that the terminal short-arm domain (domains VI and V) of the B1 chain (fragment E4) pos sesses one of the laminin self-interaction sites [Schittny, J. C. & Yu rchenco, P. D. (1990) J. Cell Biol. 110, 825-832], but that the bindin g partner(s) of this domain is unknown. Using affinity retardation chr omatography we now investigate the domain(s) fragment E4 binds to. The elution of E4 was clearly retarded on immobilized laminin and fragmen t E1' (three-chain short-arm complex excluding the distal part of the B1 chain), but not on immobilized E4 in calcium containing buffer and at 37-degrees-C. Under the same conditions, E1' strongly interacts wit h immobilized E4. In addition, E1' is able to non-covalently cross-lin k soluble E4 to immobilized E4. No further interaction of laminin and E4 with additional fragments (P1', A, B2 and B1 chain short-arm comple x without B1-domains VI-IV and without globules; E8, distal long arm a nd G1-3; E3, long-arm G subdomains 4 and 5) could be demonstrated. The se data are interpreted as evidence that (a) the primary laminin-lamin in bonds are formed between the short arms of laminin, that (b) the te rminal B1 short-arm domain (E4) can interact with the short arm(s) of the A and/or B2 chain(s) (domain E1'), but does not self-interact, and that (c) due to at least three self-binding sites, laminin polymeriza tion behaves co-operatively.