The papillomavirus E2 proteins regulate the transcription of all papillomavirus genes and are necessary for viral DNA replication. Disruption of the E2 gene is commonly associated with malignancy in cervical carcinoma, indicating that E2 has a role in regulating tumor progression. Although the E2 proteins from all characterized papillomaviruses bind specifically to the same 12-base pair DNA sequence, the cancer-associated human papillomavirus E2 proteins display a unique ability to detect DNA flexibility and intrinsic curvature. To understand the structural basis for this phenomenon, we have determined the crystal structures of the human papillomavirus-18 E2 DNA-binding domain and its complexes with high and low affinity binding sites. The E2 protein is a dimeric ␤-barrel and the E2-DNA interaction is accompanied by a large deformation of the DNA as it conforms to the E2 surface. DNA conformation and E2-DNA contacts are similar in both high and low affinity complexes. The differences in affinity correlate with the flexibility of the DNA sequence. Preferences of E2 proteins from different papillomavirus strains for flexible or prebent DNA targets correlate with the distribution of positive charge on their DNA interaction surfaces, suggesting a role for electrostatic forces in the recognition of DNA deformability.Networks of hydrogen bonds between amino acid side chains and the functional groups of DNA bases are a well documented mechanism of DNA sequence recognition by proteins. Not as well understood is the stereochemical and energetic basis by which protein-DNA binding affinity is modulated when all of these direct components of interaction are conserved. Biologically, such discriminatory abilities are essential when DNAbinding proteins have to select between multiple binding sites present among a vast excess of nonspecific DNA. This is the case in the papillomaviruses, where the primary transcriptional regulatory protein, E2, is confronted by numerous binding sites on the viral genome (Fig. 1a). These sites must be occupied in a defined order such that transcription and viral DNA replication proceed in a regulated fashion (1-7).The papillomaviruses are a large family of double-stranded DNA viruses that vary in host species specificity, tissue tropism, and the clinical outcome of infection (8, 9). The E2 protein has the same functions in the over 100 characterized papillomavirus strains: to activate or repress transcription in a contextdependent fashion and to facilitate the initiation of viral DNA replication via interactions with the viral replication protein E1. Differences in the details of transcription and replication control have been documented between the viral strains that infect different host tissues and between the papillomavirus strains that are associated with different conditions in humans such as warts or cervical cancer (2-7). The E2 proteins from all viral strains have in common the fact that they bind a palindromic DNA sequence ACCgNNNNcGGT, referred to as the E2 binding site (E2BS; 1 lowerc...

The vertebrate lens is a relatively simple cellular structure that has evolved to refract light. The ability of the lens to focus light on the retina derives from a number of properties including the expression at high levels of a selection of soluble proteins referred to as the crystallins. In the present study, we have used differential cDNA display techniques to identify a novel, highly abundant and soluble lens protein. Though related to the family of soluble lectins called galectins, it does not bind ␤-galactoside sugars and has atypical sequences at normally conserved regions of the carbohydrate-binding domain. Like some galectin family members, it can form a stable dimer. It is expressed only in the lens and is located at the interface between lens fiber cells despite the apparent lack of any membrane-targeting motifs. This protein is designated GRIFIN (galectin-related inter-fiber protein) to reflect its exclusion from the galectin family given the lack of affinity for ␤-galactosides. Although the abundance, solubility, and lens-specific expression of GRIFIN would argue that it represents a new crystallin, its location at the fiber cell interface might suggest that its primary function is executed at the membrane.

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