Precisely defined protein-polymer conjugates: construction of synthetic DNA binding domains on proteins by using multivalent dendrons

Nature has evolved proteins and enzymes to carry out a wide range of sophisticated tasks. Proteins modified with functional polymers possess many desirable physical and chemical properties and have applications in nanobiotechnology. Here we describe multivalent Newkome-type polyamine dendrons that function as synthetic DNA binding domains, which can be conjugated with proteins. These polyamine dendrons employ naturally occurring spermine surface groups to bind DNA with high affinity and are attached onto protein surfaces in a site-specific manner to yield well-defined one-to-one protein-polymer conjugates, where the number of dendrons and their attachment site on the protein surface are precisely known. This precise structure is achieved by using N-maleimido-core dendrons that selectively react via 1,4-conjugate addition with a single free thiol group on the protein surface-either Cys-34 of bovine serum albumin (BSA) or a genetically engineered cysteine mutant of Class 11 hydrophobin (HFBI). This reaction can be conducted in mild aqueous solutions (pH 7.2-7.4) and at ambient temperature, resulting in BSA- and HFBI-dendron conjugates. The protein-dendron conjugates constitute a specific biosynthetic diblock copolymer and bind DNA with high affinity, as shown by ethidium bromide displacement assay. Importantly, even the low-molecular-weight first-generation polyamine dendron (11 kDa) can bind a large BSA protein (66.4 kDa) to DNA with relatively good affinity. Preliminary gene transfection, cytotoxicity, and self-assembly studies establish the relevance of this methodology for in vitro applications, such as gene therapy and surface patterning. These results encourage further developments in protein-dendron block copolymer-like conjugates and will allow the advance of functional biomimetic nanoscale materials.