Perspectives for the Future

In this chapter, we have seen that cells have intricate mechanisms for the regulated spatial and temporal assembly and turnover of microfilaments to perform their many functions. Biochemical analyses of actin-binding proteins coupled with protein inventories provided by sequences of whole genomes have allowed the cataloging of many different classes of actin-binding proteins. To understand how this large group of proteins can assemble specific structures in a cell, it will be important to know the concentrations of all the components, how they interact, and their regulation by signaling pathways. Although this might seem like a daunting task, new microscopic methods of detecting the locations of specific protein-protein interactions and the locations of many of the key signaling pathways suggest that rapid progress will be made in this area.

The protein inventories provided by genomic sequences have also documented a large number of myosin families, yet the biochemical properties of many of these motors, or their biological functions, remain to be elucidated. Again, recent technical developments, including the ability to tag motors with fluorescent tracers such as GFP, or to knock out relevant genes with the CRISPR technology, or to knock down their expression with RNAi, are providing very powerful avenues to help reveal motor functions. However, some important aspects of motors remain largely unexplored. For example, a motor that transports an organelle down a filament first has to bind the organelle, then transport it, and then release it at the destination. However, little is known about how these different events are coordinated or how these types of myosin-based motors are returned to pick up new cargo.

Twenty years ago it was believed that all actin assembly was driven by activation of the Arp2/3 complex. Then the nucleating and capping activities of formins were discovered, and more recently, additional actin nucleators, with colorful names such as Spire, Cordon-bleu, WASH, and WHAMM have been discovered. It is likely that additional nucleators will continue to be discovered.

Finally, although we have generally discussed microfilaments without regard to tissue type—except for the specializations found in skeletal and smooth muscles—many actin-binding proteins show cell-type-specific expression, so the array and relative levels of these proteins must be tailored to the specific functions of different cell types. This conclusion is further supported by proteomic analysis of cell-specific protein expression and by the fact that many diseases are a consequence of tissue-specific expression of actin-binding proteins or myosins.