Fig. 1. De novo keratin filament polymerization. To test the capability of the EGFP-K5 fusion protein to form a functional keratin filament network, human lens cells (which otherwise do not express keratins) were transfected with either EGFP-K5/K14 pcDNA3 (A, B) or K5/K14 pcDNA3 (C,D) constructs (see Materials and Methods). At day 1 after transfection a perinuclear ring of keratin filaments is formed in both cases (A, C). At day 2, a keratin filament network is formed in lens cells transfected with the K5/K14 pcDNA3 constructs (D). Extended network formation with the EGFP-K5/K14 pcDNA3 constructs was slightly slower, but by day 3 (B), the network produced was indistinguishable from that of the K5/K14 pcDNA3-transfected cells.