The podosome cap: past, present, perspective

Podosomes are prominent actin-based adhesion structures in a variety of cell types. They feature an extensive repertoire of functions, which requires exquisite spatiotemporal fine-tuning. Accordingly, podosomes consist of hundreds of different components, which fulfill specific structural and regulatory functions. Moreover, it has become apparent that podosome architecture is more intricate than previously believed. The classical model of an actin-rich core surrounded by a ring structure containing adhesion plaque proteins thus had to be expanded, and several additional substructures have been described, most notably the podosome cap on top of the actin-rich core. Here, we discuss the known components of the podosome cap, the history of their detection and their potential regulatory roles in podosome turnover and function. We also point out strategies for identifying further cap components and present a new model for the podosome cap as a multilayered module that fine-tunes actomyosin contractility, a central requirement for podosome architecture, dynamics and function.