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HE tubulin family now includes a-and 13-tubulin, which are the main subunits of eukaryotic microtubules; -y-tubulin, which nucleates these microtubules and regulates their dynamics at the minus end (11,17); and FtsZ, a prokaryotic homolog of the tubulins that is the major cytoskeletal protein in bacterial cell division (5). Two new members, ~-and e-tubulin, have been inferred from sequences in databases (2). Both a/13-tubulin and FtsZ (6) assemble into straight protofilaments that can associate further to make two-dimensional (2-D) 1 protofilament sheets. Both types of protofilaments can also adopt a curved conformation, forming small rings or spirals. The family of tubulin rings was extended recently by the discovery that -/tubulin also forms small spirals (16,17,25). We propose here that both protofilaments and rings are formed by all members of the tubulin family, and that they are structurally homologous across the family. This inspires a new model for how "y-tubulin rings might nucleate microtubule assembly. The Lattice of the Microtubule WallThe structure of the microtubule is diagrammed in Fig. 1. The wall is a 2-D polymer of subunits connected by two types of bonds. Longitudinal bonds connect alternating c~-and 13-subunits into protofilaments and lateral bonds connect subunits in adjacent protofilaments. When a flattened microtubule wall is viewed with the protofilaments vertical, the lateral bonds form a line of subunits with a 10 ° pitch from the horizontal (Fig. 2), which forms a shallow helix in the intact microtubule. This is called a 3-start helix, because it meets the third subunit up after completing a turn, and it is necessary to start three independent helices to cover all the subunits. Lateral bonds connect primarily c~ to a and 13 to 13 (12,21,22). However this lattice cannot be continued with perfect symmetry. As shown in Fig. 1, the 3-start helix of a subunits meets a 13 subunit when it completes the circuit. The helix continues with a string of

The evolving technology of computer autofabrication makes it possible to produce physical models for complex biological molecules and assemblies. Augmented reality has recently developed as a computer interface technology that enables the mixing of real-world objects and computer-generated graphics. We report an application that demonstrates the use of autofabricated tangible models and augmented reality for research and communication in molecular biology. We have extended our molecular modeling environment, PMV, to support the fabrication of a wide variety of physical molecular models, and have adapted an augmented reality system to allow virtual 3D representations to be overlaid onto the tangible molecular models. Users can easily change the overlaid information, switching between different representations of the molecule, displays of molecular properties, or dynamic information. The physical models provide a powerful, intuitive interface for manipulating the computer models, streamlining the interface between human intent, the physical model, and the computational activity.

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