Cell cycle-regulated expression and subcellular localization of a kinesin-8 member human KIF18B

The human genome contains genes encoding for over 40 different types of kinesin and kinesin-like proteins. Of these, the functions of 13 kinesins remain uncharacterized. In this study, we constructed a plasmid containing the ORF of KIF18B and revealed that the KIF18B message of approximately 3 kb is expressed in a tissue- and cell type-specific manners. A polypeptide of 842 amino acids was deduced from the ORF sequence. We identified another form of 873 amino acids which arises from alternative splicing at the C-terminal end. We also generated an anti-KIF18B antibody which detects a protein band of 120 kDa. Western analyses showed that the protein level of KIF18B is elevated at late G2 through metaphase, very similar to cyclin B1. Immunocytochemical staining revealed that the KIF18B protein is present predominantly in the nucleus and to a lesser extent in the cytoplasm of interphase cells. During mitosis, most KIF18B was found to be closely associated with astral microtubules emanating from the spindle pole during prometaphase and metaphase. Meanwhile, KIF18B was not detected at anaphase and telophase, consistent with the Western blotting data. The nuclear localization signal was roughly determined by using several EGFP-tagged deletion mutants of KIF18B. Together, the expression of KIF18B is regulated in a cell cycle-dependent manner and therefore may play an important role(s) in cell division.