Distinct roles of mouse laminin beta1 long arm domains for alpha1beta1gamma1 trimer formation

Mouse embryonal carcinoma F9 cells expressing partial mouse laminin beta1 covering either the C-terminal end (delta beta1S) or the whole (delta beta1L) of the long arm were established to study the assembly and interchain disulfide-bonding of beta1 to endogenous laminin alpha1 and gamma1. Both delta beta1S and delta beta1L were disulfide-bonded to gamma1 but only delta beta1L gamma1 dimer formed a disulfide-bonded alpha1 delta beta1L gamma1 trimer which was actively secreted into the medium. Meanwhile, in the cells producing delta beta1S gamma1 dimer, the level of endogenous alpha1 beta1 gamma1 was reduced but the level of monomeric alpha1 was increased, suggesting that alpha1 was recruited to trimer formation with the delta beta1S gamma1 dimer without disulfide-bonding. This shows that the delta beta1S gamma1 dimer can associate with alpha1 but not support the disulfide-bonding at the N-terminus of the long arm of alpha1. While control cells secrete neither monomeric alpha1 nor the beta1gamma1 dimer into the medium, the delta beta1S gamma1 producing cells probably do as alpha1 delta beta1 gamma1 trimer. We thus propose that the N- and C-termini of the long arm of laminin beta1 have distinct roles for trimer formation.