CacyBP/SIP interacts with tubulin in neuroblastoma NB2a cells and induces formation of globular tubulin assemblies.

CacyBP/SIP, originally identified as a S100A6 (calcyclin) target, was later shown to interact with some other members of the S100 family as well as with Siah-1 and Skp1 proteins. Recently, it has been shown that CacyBP/SIP is up-regulated during differentiation of cardiomyocytes. In this work we show that the level of CacyBP/SIP is higher in differentiated neuroblastoma NB2a cells than in undifferentiated ones and that in cells overexpressing CacyBP/SIP the level of GAP-43, a marker of differentiation, was increased. Since the process of differentiation is accompanied by an extensive rearrangement of microtubules, we examined whether CacyBP/SIP interacted with tubulin. By applying cross-linking experiments we found that these two proteins bind directly. The dissociation constant of the tubulin-CacyBP/SIP complex determined by the surface plasmon resonance technique is 1.57 x 10(-7 )M which suggests that the interaction is tight. The interaction and co-localization of CacyBP/SIP and tubulin was also demonstrated by co-immunoprecipitation, affinity chromatography and immunofluorescence methods. Light scattering measurements and electron microscopy studies revealed that CacyBP/SIP, but not its homologue, Sgt1, increased tubulin oligomerization. Altogether, our results suggest that CacyBP/SIP, via its interaction with tubulin, might contribute to the differentiation of neuroblastoma NB2a cells.