Sulfolobus turreted icosahedral virus (STIV) was isolated from acidic hot springs of Yellowstone National Park and was the first hyperthermophilic virus described with icosahedral capsid architecture. Structural analysis of the STIV particle and its major capsid protein suggests that it belongs to a lineage of viruses that predates the division of the three domains of life. Functional predictions of the viral proteins are hindered because they lack similarity to sequences of known function. Protein structure, however, may suggest functional relationships that are not apparent from the sequence. Thus, we have initiated crystallographic studies of STIV and expect to gain functional insight into its proteins while illuminating the viral life cycle. These studies may also provide genetic, biochemical, and evolutionary insight into its thermoacidophilic host and the requirements for life in these harsh environments. The first three proteins studied in structural detail are A197, B116, and F93. As anticipated, these structures suggest possible functions. The structure of A197 reveals a glycosyltransferase GT-A fold.Within the context of the GT-A fold, are the canonical DXD motif and a putative catalytic base, hallmarks of this family of enzymes, strongly suggesting glycosyltransferase activity for A197. B116 is a unique structure that lacks significant homology to known protein structures. However, sequence similarity to proteins from other hyperthermophilic viruses reveals conserved surface features suggesting interaction with a host macromolecule, likely DNA. The F93 structure reveals a winged-helix fold common among DNA-binding proteins, in particular, the MarR-like family of transcriptional regulators. The most likely role for F93 is thus regulation of viral transcription. Interestingly, B116 contains an intramolecular disulfide bond while F93 contains an intermolecular disulfide bond. The presence of these disulfide bonds was not anticipated because these proteins are expected to be localized within the host cell. This prompted analysis of the cysteine distribution in the STIV genome, which suggests that disulfide bonds are common in intracellular (cytoplasmic) proteins encoded by STIV. This work is in accordance with accumulating evidence that disulfide bonds are common stabilizing elements in the intracellular proteins of thermophilic organisms in general, and extends the observation to genomes of hyperthermophilic viruses.