The role of copines in the regulation of the actin cytoskeleton and membrane trafficking

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PROJECT SUMMARY Copines are a family of calcium-dependent membrane binding proteins found in most eukaryotic organisms including humans, which have nine different copine genes. Copines have been implicated in numerous human cancers. However, at this time there is no unifying theme with respect to the function of these elusive proteins. The main goal of this research project is to determine whether a common basic mechanistic function can be attributed to all copine proteins. We have been using the model organism Dictyostelium discoideum to study these enigmatic proteins. Dictyostelium is a good model organism to study these proteins because they have six copines genes, while other model organisms either have a few or no copine genes. Our studies have mostly focused on Copine A (CpnA). However, we now have many of the tools to study the other five copines (CpnB-F). Our studies on cpnA knockout mutants in Dictyostelium indicate that CpnA is involved in many cellular functions including processes that require the actin cytoskeleton (i.e. cytokinesis, chemotaxis, cell polarity, and adhesion) and require membrane fusion (i.e. contractile vacuole expulsion, and postlysosome maturation and exocytosis). Biochemical studies indicate that CpnA binds to acidic phospholipids and actin filaments in a calcium-dependent manner. Localization studies indicate that CpnA is a soluble cytoplasmic protein that translocates to the plasma membrane and the membranes of the contractile vacuole system and organelles of the endolysosomal system in response to a rise in calcium concentration. Two main hypotheses for the function of CpnA emerge from our studies: CpnA functions in the calcium-dependent regulation of 1) actin filament dynamics and/or 2) membrane fusion. Therefore, we propose to use Dictyostelium to explore the idea that all copines, from single-celled organisms to humans, are involved in the calcium-dependent regulation of actin filament dynamics and/or membrane fusion. We plan to use several strategies that include the functional characterization of copine knockout mutants, identification of copine protein binding partners, and fluorescence microscopy techniques to visualize actin filament dynamics and membrane fusion in the copine knockout mutants. We will also explore the more specific hypothesis that copines regulate actin filaments on membrane surfaces to regulate membrane fusion. If we find that not all copines function in the regulation of actin filament dynamics and/or membrane fusion, a unifying theme for copines will most likely emerge as we characterize each of the copine knockout mutants to identify any common defects and identify common binding-partners of copine proteins. This new knowledge can be then be used to understand the molecular mechanisms underlying many of the human cancers in which copines are highly expressed.