Abstract
Cells are the fundamental unit of life and make up all the tissues in the body. Just like organs in our body, organelles in cells specialize in a specific function and play a vital role in their survival. The cytoskeleton, one such organelle, extends from the nucleus to the cell membrane performing a multitude of cellular functions: giving cells shape and mechanical resistance to deformation, motility, cell signaling, endocytosis, intracellular transport, segregation of chromosome during cell division, and cytokinesis. Actin is a key cytoskeletal protein and one of the most abundant protein in eukaryotic cells. Actin polymerizes into filaments that bundle into linear structures in vivo such as filopodia at the leading edge of motile cells, stereocilia in inner and outer hair cell bundles, microvilli on the apical surfaces of epithelial cells, and actin cables in budding and fission yeast cells. These structures in vivo turnover rapidly, yet are often maintained at a specific length crucial to realize their cellular functions. Their dynamic nature is exploited in “balance point models” where the assembly or disassembly rates are assumed to be tuned in a length-dependent manner. These models are widely used to explain the length-dependent growth of cytoskeletal filaments. First, we show that these actin structures exhibit a universal scaling of the variance with the square of their mean length in contradiction to the predictions of the balance point model. We propose a model where accounting for the multiple filament nature of these structures can lead to length control even when individual filaments are unregulated. Second, we quantify the distribution of actin associated proteins in wildtype and mutant cells with twice the length of former. Yeast cells build and maintain the length of their actin cables for variable cell lengths. We find a gradient of depolymerizing factor, Srv2 on both the poles of the cell scaling with the length of the cell. This cell size dependent gradient could be a possible mechanism by which cells can scale their cytoskeletal filaments to the linear dimensions of the cell. In addition to scaling gradients, we also find that the distribution is dependent on the actin cytoskeleton itself. This illustrates an amazing cell assembly pathway for organelle length control and scaling where the feedback from the structure and proteins that disassemble these structures could co-regulate each other.