Structure and Function of Non-Conventional Caveolins

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Caveolins are a family of unusual membrane proteins that function as key regulators of the cardiovascular system and metabolism. One of their major biological activities is to shape the plasma membrane to form flask-shaped invaginations called caveolae. Defects in caveolins and caveolae have dramatic physiological consequences and disrupt intracellular trafficking, signaling, lipid homeostasis, mechanosensing, and plasma membrane integrity at the cellular level. How caveolins and caveolae regulate so many different cellular functions has remained a mystery for nearly 30 years, in part due to the lack of information about the structure of caveolins. Excitingly, the status quo recently changed. Using cryo-electron microscopy, we have now determined the first high-resolution structure of the caveolin family member responsible for caveolae biogenesis outside of muscle, caveolin-1 (CAV1). Consisting of 11 tightly packed protomers arranged in a disc, the structure represents an oligomeric state of the protein that serves as the fundamental building block of caveolae. It is thus now possible to begin to address how caveolae form and function at a mechanistic level. Here, we propose to build on lessons learned from determining the structure of CAV1 to tackle another ongoing conundrum in the field. Either as a consequence of disease-associated mutations or as a result of natural selection, some caveolins are unable to generate caveolae on their own. Remarkably, these “non- conventional” caveolins can still have profound effects on caveolae assembly and dynamics and even exert distinct biological functions. How does this happen? To gain insight into this long-standing question, we propose to compare and contrast the properties of CAV1 with caveolin-2 (CAV2), an evolutionarily conserved, naturally occurring example of a caveolin that can only form caveolae in the presence of CAV1 and is required for normal physiological function of the lung. Using a combination of structural, biochemical, biophysical, computational, and cell biological assays, we will 1) determine how the unique structural features of CAV2 dictate its interactions with itself, CAV1, and other proteins and 2) study mechanisms used by caveolin complexes to associate with and bend membranes and mediate plasma membrane homeostasis. These studies will provide critical insights into how caveolins interact with themselves and one another to form the building blocks of caveolae as well as how the distinct structural features of caveolin family members dictate their biological functions by controlling their repertoire of interacting proteins and lipids. On a more fundamental level, the proposed investigations will test new ideas about how proteins insert into membranes and how this influences their ability to mold membrane morphology, composition, and function.