The rotational order-disorder structure of the reversibly photoswitchable red fluorescent protein rsTagRFP

Identifying and Overcoming Crystal Pathologies: Disorder and Twinning

Macromolecular crystals are prone to a number of pathologies that result from aberrant molecular packing. Two common pathologies encountered in macromolecular crystals are rigid-body disorder and twinning. When a crystal displays one of these pathologies, its diffraction pattern is altered in a way that generally complicates structure determination. The severity of the underlying abnormalities varies from case to case, and sometimes the resulting alterations to the diffraction pattern are immediately obvious, while at other times they may go entirely unnoticed. Structure determination from a crystal that suffers from disorder or twinning may or may not be possible, depending on the specific nature of the pathology, and on how the data are handled. This chapter provides an introduction to these pathologies, with an emphasis on providing guidelines for identifying and overcoming them when they pose a threat to successful structure determination.

The Antiviral and Cancer Genomic DNA Deaminase APOBEC3H Is Regulated by an RNA-Mediated Dimerization Mechanism

Human APOBEC3H and homologous single-stranded DNA cytosine deaminases are unique to mammals. These DNA-editing enzymes function in innate immunity by restricting the replication of viruses and transposons. APOBEC3H also contributes to cancer mutagenesis. Here, we address the fundamental nature of RNA in regulating human APOBEC3H activities. APOBEC3H co-purifies with RNA as an inactive protein, and RNase A treatment enables strong DNA deaminase activity. RNA-binding-defective mutants demonstrate clear separation of function by becoming DNA hypermutators. Biochemical and crystallographic data demonstrate a mechanism in which double-stranded RNA mediates enzyme dimerization. Additionally, APOBEC3H separation-of-function mutants show that RNA binding is required for cytoplasmic localization, packaging into HIV-1 particles, and antiviral activity. Overall, these results support a model in which structured RNA negatively regulates the potentially harmful DNA deamination activity of APOBEC3H while, at the same time, positively regulating its antiviral activity.