Bridging cytoskeletal intersections

The cells of multicellular organisms come in fascinating shapes and sizes that are nearly as varied as the animals themselves. Cytoarchitecture is responsible for this diversity and also contributes greatly to the multifaceted functions of each cell type. Historically, the three major cytoskeletal networks—actin microfilaments, intermediate filaments (IFs), and microtubules—were viewed as independent entities, and cytoskeletal researchers focused on the most important functions ascribed to each of these networks. From these initial studies, actin microfilaments were found to play pivotal roles in cell polarity and contractile and migratory processes. In contrast, IFs impart intracellular mechanical strength and are consequently especially abundant in tissues such as epidermis and muscle that undergo substantial physical stress. Cytoplasmic microtubules are vital for intracellular trafficking of vesicles, organelles, and proteins, whereas spindle microtubules function in the dynamics of chromosome alignment and segregation during mitosis.

As cell biologists searched for functional differences between cytoskeletal networks and identified proteins specifically aligned with each network, they discovered that agents perturbing one filamentous network often affected the others and that functions ascribed to one network were sometimes also features of another network. In the past 5 yr, it has become increasingly clear that multiple cytoskeletal networks cooperate to perform many of their tasks. Facilitating these close encounters are connector proteins that have dual capacities to associate with more than one cytoskeleton, enabling them to integrate old functions and generate new ones. Many of the proteins initially found to associate with microtubules, and hence called microtubule-associated proteins (MAPs), have been found to also associate with actin microfilaments. A number of microtubule-based motor proteins (kinesins and dynein) and actin-based motor proteins (myosins) have now been found to be members of multiprotein, bifunctional motor complexes that can maneuver along multiple cytoskeletal roadways to transport common cargo. One of the biggest surprises in …