Significance Chloroplast biogenesis is a fundamental process occurring during seedling ontogenesis and leading to plant autotrophy. Which membrane components sterically organize the light-triggered transition of etioplast prolamellar bodies (PLBs) into chloroplast thylakoids, and thus mediate cubic–lamellar transformation, is poorly understood. Here, we used combined two- and three-dimensional electron microscopy, spectroscopy, and biochemical methods to determine the role of CURT1 proteins in the formation of etioplast cubic membranes and their transformation to photosynthetically active chloroplast thylakoids. CURT1 proteins were previously recognized as significant contributors to thylakoid membrane folding. We found that CURT1 proteins are integral proteins of etioplast membranes and act as factors modulating PLBs and prothylakoid nanomorphology. They are also required for concerted thylakoid maturation under de-etiolation.