The 14-3-3 Proteins of Trypanosoma brucei Function in Motility, Cytokinesis, and Cell Cycle

The cDNAs for two isoforms (I and II) of the 14-3-3 proteins have been cloned and functionally characterized in Trypanosoma brucei. The amino acid sequences of isoforms I and II have 47 and 50% identity to the human tau isoform, respectively, with important conserved features including a potential amphipathic groove for the binding of phosphoserine/phosphothreonine-containing motifs and a nuclear export signal-like domain. Both isoforms are abundantly expressed at approximately equal levels (1-2 x 10(6) molecules/cell) and localized mainly in the cytoplasm. Knockdown by induction of double-stranded RNA of isoform I and/or II in both bloodstream and procyclic forms resulted first in a reduction of cell motility and then significant reduction in cell growth rates and morphological changes; the changes include aberrant numbers of organelles and abnormal shapes and sizes that mimic phenotypes produced by various cytokinesis inhibitors. Morphological and fluorescence-activated cell sorting analysis of the cell cycle suggested that isoforms I and II might play important roles in nuclear (G2-M transition) and cell (M-G1 transition) division. These findings indicate that the 14-3-3 proteins play important roles in cell motility, cytokinesis, and the cell cycle.