Naf1 alpha is an HIV Nef-associated factor expressed ubiquitously in human cells. Previously, we reported that Naf1 alpha is phosphorylated with EGF through MEK/ERK2 pathway. In this study, we found an additional phosphorylation of Naf1 alpha when cells are in mitotic phase (M phase) or arrested in M phase with anti-mitosis reagents, and disappeared when the cells exit from mitotic phase to G1 phase. Furthermore, we demonstrated that Naf1 alpha plays an important role in preventing cells from apoptosis: overexpression of Naf1 alpha in Saos-2 cells suppressed trichostatin A (TSA)-induced apoptosis either of random culture or of cell population synchronized in M phase. In addition, knock-down of Naf1 alpha expression with small interfering RNA sensitized Saos-2 cells to TSA-induced apoptosis. Physiological significance of these findings is discussed in relation to protection of cells from the apoptosis induction. (c) 2007 Elsevier Inc. All rights reserved.