Microtubule-Organizing Centers: Towards a Minimal Parts List

Despite decades of molecular analysis of the centrosome, an important microtubule-organizing center (MTOC) of animal cells, the molecular basis of microtubule organization remains obscure. A major challenge is the sheer complexity of the interplay of the hundreds of proteins that constitute the centrosome. However, this complexity owes not only to the centrosome’s role as a MTOC but also to the requirements of its duplication cycle and to various other functions such as the formation of cilia, the integration of various signaling pathways, and the organization of actin filaments. Thus, rather than using the parts lists to reconstruct the centrosome, we propose to identify the subset of proteins minimally needed to assemble a MTOC and to study this process at non-centrosomal sites. The centrosome, besides being a microtubule-organizing center (MTOC), is a multifunctional organelle with roles in signaling, formation of cilia, actin organization, and centriole duplication. The majority of centrosome proteins have been identified, but the subset of proteins that are directly involved in organizing microtubules remains poorly defined. Currently available data suggest that the centrosome may comprise different types of microtubule-nucleation and -anchoring sites and a large number of regulatory proteins. The centrosome may not be an ideal model MTOC due to its inherent compositional, structural, and functional complexity. Analysis of non-centrosomal MTOCs has suggested that the list of proteins minimally required to assemble a functional MTOC is much shorter than that suggested by the centrosome proteome.