As the primary microtubule-organizing centers, centrosomes require ␥-tubulin for microtubule nucleation and organization. Located in close vicinity to centrosomes, the Golgi complex is another microtubule-organizing organelle in interphase cells. CDK5RAP2 is a ␥-tubulin complex-binding protein and functions in ␥-tubulin attachment to centrosomes. In this study, we find that CDK5RAP2 localizes to the Golgi complex in an ATPand centrosome-dependent manner and associates with Golgi membranes independently of microtubules. CDK5RAP2 contains a centrosome-targeting domain with its core region highly homologous to the Motif 2 (CM2) of centrosomin, a functionally related protein in Drosophila. This sequence, referred to as the CM2-like motif, is also conserved in related proteins in chicken and zebrafish. Therefore, CDK5RAP2 may undertake a conserved mechanism for centrosomal localization. Using a mutational approach, we demonstrate that the CM2-like motif plays a crucial role in the centrosomal and Golgi localization of CDK5RAP2. Furthermore, the CM2-like motif is essential for the association of the centrosome-targeting domain to pericentrin and AKAP450. The binding with pericentrin is required for the centrosomal and Golgi localization of CDK5RAP2, whereas the binding with AKAP450 is required for the Golgi localization. Although the CM2-like motif possesses the activity of Ca 2؉ -independent calmodulin binding, binding of calmodulin to this sequence is dispensable for centrosomal and Golgi association. Altogether, CDK5RAP2 may represent a novel mechanism for centrosomal and Golgi localization.

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A unified Mie and fractal model for light scattering by biological cells is presented. This model is shown to provide an excellent global agreement with the angular dependent elastic light scattering spectroscopy of cells over the whole visible range (400 to 700 nm) and at all scattering angles (1.1 to 165 deg) investigated. Mie scattering from the bare cell and the nucleus is found to dominate light scattering in the forward directions, whereas the random fluctuation of the background refractive index within the cell, behaving as a fractal random continuous medium, is found to dominate light scattering at other angles. Angularly dependent elastic light scattering spectroscopy aided by the unified Mie and fractal model is demonstrated to be an effective noninvasive approach to characterize biological cells and their internal structures. The acetowhitening effect induced by applying acetic acid on epithelial cells is investigated as an example. The changes in morphology and refractive index of epithelial cells, nuclei, and subcellular structures after the application of acetic acid are successfully probed and quantified using the proposed approach. The unified Mie and fractal model may serve as the foundation for optical detection of precancerous and cancerous changes in biological cells and tissues based on light scattering techniques.