Depletion of the Ras association domain family 1, isoform A-associated novel mitochondrial associated protein, C19)RF5/MAP1S, causes mitotic abnormalities

Ras association domain family 1, isoform A (RASSF1A) is a novel tumor suppressor gene that is found to be inactivated in more than 40 types of sporadic cancers. In addition, mouse Rassf1a knockout models have an increased frequency of spontaneous and induced tumors. The mechanisms by which RASSF1A exerts its tumor suppression activities or the pathways it can regulate are not yet fully understood. Using yeast two-hybrid system, we have previously identified C190RF5/MAP1S as the major RASSF1A-interacting protein. C190RF5 has two conserved microtubule-associated regions and may function to anchor RASSF1A to the centrosomes. In this study, we have analyzed the cellular functions of C190RF5. By using small interfering RNA-mediated depletion and time-lapse video microscopy, we show that C190RF5 knockdown causes mitotic abnormalities that consist of failure to form a stable metaphase plate, premature sister chromatid separation, lagging chromosomes, and multipolar spindles. We also show that a fraction of C190RF5 localizes to the spindle microtubules. Additionally, we show here that C190RF5 localizes to the microtubule-organizing centers during microtubule regrowth after nocodazole washout. Knockdown of C190RF5 disrupts the microtubule-organizing center and results in microtubule nucleation from several sites. Whereas the localization of pericentrin is not affected, alpha- and gamma-tubulin localization and sites of nucleation are greatly altered by C190RF5 depletion. This may indicate that C19ORF5 plays a role in anchoring the microtubule-organizing center to the centrosomes. In addition, we show that the NH2 terminus of C19ORF5 is essential for this process. This novel role for C190RF5 could explain the resulting mitotic abnormalities that occur on its depletion and can potentially provide an underlying mechanism for the frequent centrosome and microtubule abnormalities detected in several cancers.