Cotranslational assembly of some cytoskeletal proteins : implications and prospects

The cytoskeleton is a complex three-dimensional web in the interior of eucaryotic cells. The cytoskeleton orders many structures in the cell and performs many kinds of transport and motility for the cell. It is distinguished by a high degree of spatial differentiation and anisotropy. How does the cell construct something so heterogeneous and complex? A widely used model of protein assembly postulates that proteins are synthesized on ribosomes and released into solution within the cell, before diffusing within the cell and assembling into cytoskeletal or other structures. This model is clearly appropriate for a variety of cytoskeletal proteins, including tubulin and actin. Because these proteins assemble after translation, much has been learned about their assembly by using in vitro protein chemistry and in vivo methods such as FRAP (fluorescence recovery after photobleaching). However, a growing body of evidence supports a different model of assembly for certain cytoskeletal proteins. Several observations, made with a variety of experimental protocols, suggest that assembly of some cytoskeletal structures is difficult to explain solely by post-translational assembly. Rather, these proteins undergo cotranslational assembly; they first associate with the cytoskeleton during translation, as nascent (not yet complete) peptides. The range of this evidence includes in situ autoradiography of cytoskeletal proteins, immunoprecipitation of nascent peptides, immunofluorescence and, most recently, fluorescent in situ hybridization.