Assessment of electron crystallographic data obtained from two-dimensional crystals of biological specimens

Over the past few years, an increasing number of electron crystallographic studies using two-dimensional crystals have shed light on the structure of biologically important macromolecules. Steady progress in the development of specimen-preparation techniques and image-processing tools enable researchers to achieve resolutions in the range of 5-10 A almost routinely. However, reaching near-atomic resolution remains a formidable task that is likely to require several years. Without doubt, this process will become far less time-consuming as methods are improved further. However, the immediate future is more likely to be dominated by structures solved to an intermediate level of resolution. Since the reliability of such structures is more difficult to assess than that of density maps at near-atomic resolution and as the popularity of electron cryo-microscopy increases it becomes more important to define standardized criteria for the evaluation of electron crystallographic data. This article discusses some of the relevant issues with the aim of stimulating further discussion about the assessment and presentation of electron crystallographic data.