Heterofunctional crosslinking groups are provided having the formula: ##STR00001## wherein W is a covalent core component; L.sup.1, L.sup.2 and L.sup.3 are independently linking groups; X is a non-covalent or reversibly covalent protein tag binder; Y is a activatable covalent linking group; and Z is a protected or unprotected covalent crosslinking group.