Abstract
We recently described the identification of a centrosome/spindle pole associated protein, CSPP, involved in cell cycle progression. Here we report a CSPP isoform denoted CSPP‐L, with a 294 amino acids longer N‐terminus and a 51 amino acids insertion located in the coiled‐coil mid‐domain. Expression analysis indicates an inverse cell cycle dependent regulation. CSPP mRNA expression is highest in G~1~ whereas CSPP‐L expression is highest in G~2~/M. Ectopic expression of CSPP‐L impairs cell cycle progression weaker in G~1~ than CSPP. Furthermore, normal mitotic phenotypes were observed in CSPP‐L but not in CSPP transfectants. CSPP‐L relocates from spindle microtubules and poles in metaphase to the mid‐spindle in anaphase and concentrates at the mid‐body in telophase/cytokinesis. CSPP‐L high‐expressing mitotic cells were predominantly characterized by lagging chromosomes or monopolar spindles, in contrast to the predominant multipolar spindles observed with CSPP expression. The different effects of CSPP and CSPP‐L on microtubule organization in mitosis depend on the coiled‐coil mid‐domain insertion. The common C‐terminal domain is required to repress that activity until mitosis. Notably, this C‐terminal domain alone can associate with centrosomes in a microtubule independent manner. Taken together, CSPP and CSPP‐L interact with centrosomes and microtubules and can differently affect microtubule organization. J. Cell. Physiol. 209: 199–210, 2006. © 2006 Wiley‐Liss, Inc.