Proteostatic regulation of caveolins avoids premature oligomerisation and preserves ER homeostasis

Abstract

Caveolin-1 (CAV1) and CAV3 are membrane sculpting proteins driving formation of plasma membrane caveolae. Caveola formation is unique as it requires oligomerisation of newly synthesised caveolins through the biosynthetic-secretory pathway. Here, we combine structural, biochemical, and microscopy analyses to examine the early proteostasis of caveolin family members and mutants. We describe striking trafficking differences between newly synthesised caveolins, with CAV1 rapidly exported to the Golgi but CAV3 showing ER retention and targeting to lipid droplets. Only monomeric/low oligomeric caveolins are efficiently exported from the ER, with oligomers assembling beyond the cis-Golgi and disease-causing mutations leading to detrimental non-functional complexes. Caveolins in the ER are maintained at low levels by active proteasomal degradation, avoiding premature oligomerisation and ER stress. Increasing lipid availability, cholesterol for CAV1 and fatty acids for CAV3, enhances trafficking and reduces proteasomal degradation. In conclusion, we identify proteostatic mechanisms that modulate stability and trafficking of newly synthesised caveolins, protecting cells against ER stress but perturbed in caveolin-related disease. SummaryUnderstanding the unique proteostasis of caveolins has important implications for cell biology and physiopathology. Combining structural, microscopy, and biochemical analyses, we uncover new insights into the mechanisms that differentiate the early biosynthetic steps of caveolin family members, isoforms, and pathogenic mutants.