Construction of mutant CycT1 proteins. A. Structure of the cyclin box repeat domain (1–281) of CycT1. Two repeats of five α-helices each form the conserved cyclin box (blue). Flanking N- and C-terminal helices, which are important for the specificity of cyclins, are depicted in yellow and red, respectively. B. Schematic representation of C-terminally truncated wt CycT1 and the dominant negative CycT1-U7 mutant used in this study. Secondary structure of conserved α-helices (dotted regions in cyclin box 1 and hatched regions in cyclin box 2) together with two helices at N- and C-terminal (gray) locate in the N-terminal cyclin boxes in CycT1. Random mutations were introduced into the nine most conserved regions (shown by thin lines) in the cyclin box domain of a C-terminal truncation mutant of CycT1 (CycT1(1–280)). "-" in the CycT1-U7 sequence represents a deletion site. The truncated wt and mutant CycT1 employed in this study are also shown. C. A schematic representation of the full-length Cyclin T1. Amino acid motifs such as cyclin boxes, Tat-TAR recognition motif (TRM), coiled-coiled region, and PEST sequence are depicted.