The binding activity of a molecule that functions as a physical support for the assembly of a multiprotein mitogen-activated protein kinase (MAPK) complex. Binds multiple kinases of the MAPKKK cascade, and also upstream signaling proteins, permitting those molecules to function in a coordinated way. Bringing together multiple enzymes and their substrates enables the signal to be transduced quickly and efficiently. [ http://www.ncbi.nlm.nih.gov/pubmed/9405336 http://www.ncbi.nlm.nih.gov/pubmed/12511654 http://www.ncbi.nlm.nih.gov/pubmed/15213240 ]