The cellular cytoskeleton relies on structural elements formed from F-actin, a highly charged, stiff biopolymer, linked together by actin-binding proteins (linkers) that can bind two actin filaments together. The resulting structures can have very different morphologies, depending on the concentrations of biopolymers and linkers. In crawling cells such as blood platelets, F-actin primarily assembles into a network but can alternatively assemble into bundles; this is important to cell motility and adhesion. This suggests that the system could be near a phase transition between networks and bundles. I will present a theoretical study that elucidates the physical mechanisms that control these structures and their stability. The study is based on a generalized Onsager theory that includes linker-mediated attractions between charged filaments, and that yields phase diagrams as a function of actin concentration and linker concentration.