Investigations of high molecular weight recombinant protein triblock copolymers demonstrate unique opportunities to systematically modify material microstructure on both nano- and mesolength scales in a manner not been previously demonstrated for protein polymer systems. Significantly, through the biosynthesis of BAB-type copolymers containing flanking, plastic-like end blocks and an elastomeric midblock, virtually cross-linked protein-based materials were generated that exhibit tunable properties in a manner completely analogous to synthetic thermoplastic elastomers. Through the rational choice of processing conditions that control meso- and nanoscale structure, changes of greater than 3 orders of magnitude in Young's modulus (0.03-35 MPa) and 5-fold in elongation to break (250-1300%) were observed. Extensibility of this range or magnitude has not been previously reported for virtually cross-linked copolymers that have been produced by either chemical or biosynthetic approaches. We anticipate that these versatile protein-based thermoplastic elastomers will find applications as novel scaffolds for tissue engineering and as new biomaterials for controlled drug release and cell encapsulation.